Calcium and natural organic matter

Divalent cations, particularly calcium, have been shown to enhance fouling of membranes with natural organic matter (NOM) including humic, fulvic, and tanic acids. Because is it s acidic nature, NOM can form complexes with dissolved metal ions. The strongest bonds occur with calcium. 

Factors that affect fouling with NOM-calcium complexes include permeate flux and cross flow rate. At higher flux though the membrane, the concentration of calcium increases in the concentration polarization boundary layer at the membrane surface, as described above. Lower cross flow rates also increase the concentration of calcium in the bormdary layer. The increases concentration of calcium at the membrane surface enhances the fouling of the membranes by the NOM-calcium aggregates.  

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Fluoride is a natural component of most types of soil, in which it is mainly bound in complexes and not readily leached. The major source of free fluoride ion in soil is the weathering and dissolution of fluoride rich rock that depends on the natural solubility of the fluoride compound in question, pH, and the presence of other minerals and compounds and of water. The major parameters that control fluoride fixation in soil through adsorption, anion exchange, precipitation, formation of mixed solids and complexes are aluminium, calcium, iron, pH, organic matter and clay [19,20]. 

Mitterer R.M. and Cunningham R Jr. (1985) The interaction of natural organic matter with grains surfaces Implications for calcium carbonate precipitation. In Carbonate Cements (eds. N. Schneidermann and P.M. Harris), pp. 17-31. Soc. Econom. Paleontoligists and Mineralogists, Tulsa, OK. 

Chlorine is applied as chlorine gas, powdered calcium hypochlorite (Ca(OCl)2), or liquid sodium hypochlorite (NaOCl bleach). Chlorine reacts with the organic (natural organic matter, NOM) or inorganic (bromide ion, Br ) precursors in the water to form chlorine disinfection by-products (CBPs), including trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), haloketones, chloral hydrate, and chloropicrin. Humic and fulvic acids are the predominant NOMs. When bromine exists, the chlorine oxidizes it to hypobromous acid/ hypobromite ion (HOBr/OBr ) to form bromo THMs (bromodichloromethane, BDCM, and di-bromochloromethane, DBCM), HAAs, and HANs. 

The presence of calcium and humic substances or natural organic matter (NOM) in surface waters can cause severe fouling of nanojittration (NF) membranes. Conditions offouling were studied as a function of solution chemistry, organic type, calcium concentration, hydrodynamic conditions, and membrane type. 

NF membranes have been used successfully for ground-water softening since they achieve greater than 90% rejection of divalent ions such as calcium and magnesiiun. NF membranes are also capable of removing greater than 90% of natural organic matter present in the water. Therefore, they are also excellent candidates for the removal of color, and also DBF precursor material. 

Well water. This is rain water which has percolated through various strata until it enters an underground aquifer. Well water usually contains dissolved calcium and magnesium salts, but is low in organic matter owing to natural filtration. 

Fusion with Alkali and Other Agents. Calcium peroxide was added portionwise to the fusion mixture at 180°C. but did not change materially the nature of the yields or reaction products. It should be noted that, in these fusions, total recovery of solids was quite low owing to large conversion of organic matter to carbon dioxide. 

Chemical treatment of natural waters. Both directly and indirectly, the general problem of purification and treatment of natural waters is related to the chemical and physical properties of the normal and acid salts of carbonic acid. The common impurities in natural waters consist of suspended solid organic and inorganic materials and of certain dissolved salts, particularly the acid carbonates, chlorides, and sulfates of sodium, calcium, and magnesium. The solid matter may be removed by filtration, the presence of limited quantities of sodium salts is not objectionable, and the calcium and magnesium salts are eliminated only through appropriate chemical treatment. The ions that are most... 

The most complete study of the inhibition of calcium carbonate precipitation by organic matter was carried out by Berner et al. (1978), where primary concern was the lack of carbonate precipitation from supersaturated seawater. Both synthetic organic compounds and organic-rich pore waters from Long Island Sound were used to measure the inhibition of aragonite precipitation. Natural marine humic substances and certain aromatic acids were found to be the strongest inhibitors. The rate of precipitation in pore waters was also found to be strongly inhibited. 

Trivalent chromium compounds, except for acetate, nitrate, and chromium(III) chloride-hexahydrate salts, are generally insoluble in water. Some hexavalent compounds, such as chromium trioxide (or chromic acid) and the ammonium and alkali metal (e.g., sodium, potassium) salts of chromic acid are readily soluble in water. The alkaline metal (e.g., calcium, strontium) salts of chromic acid are less soluble in water. The zinc and lead salts of chromic acid are practically insoluble in cold water. Chromium(VI) compounds are reduced to chromium(III) in the presence of oxidizable organic matter. However, in natural waters where there is a low concentration of reducing materials, chromium(VI) compounds are more stable (EPA 1984a). For more information on the physical and chemical properties of chromium, see Chapter 3. 

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