Trace Metals—Barium and Strontium

Barium and strontium form sulfate scales that are not readily soluble. In fact, barium is the least soluble of all the alkaline-earth sulfates. It can act as a catalyst for strontium and calcium sulfates scale.4 Analyses of the ion product with the solubility constants for barium and strontium sulfates is necessary to determine the potential for scaling with these species. If the ion product (IP) for barium sulfate exceeds the solubility constant, scale will form. Note that in the case of strontium sulfate, if IP 0.8Ksp, scaling is likely. However, the induction period (the time it takes for scale to form) is longer for these sulfate-based scales than it is for calcium carbonate scale. 

Barium and strontium can be reduced in RO feed water using sodium softening (see Chapter 8.1.6). Antisealant can be used to control or inhibit scaling without reducing the concentration of either species (see Chapter 8.2.3). 

Barium and strontium form sulfate scales that are not readily soluble. In fact, barium is the least soluble of all the alkaline-earth sulfates. It can act as a catalyst for strontium and calcium sulfate scales. Analyses of the ion product with the solubility constants for barium and strontium sulfates is 

Polyamide, composite membranes are very sensitive to free chlorine (recall from Chapter 4.2.1 that cellulose acetate membranes can tolerate up to 1 ppm free chlorine continuously) while polyamide membranes cannot tolerate chlorine (4.2.2.2)). Degradation of the polyamide composite membrane occurs almost immediately upon exposure and can result in significant reduction in rejection after 200 and 1,000-ppm hours of exposure to free chlorine (in other words after 200-1,000 hours exposure to 1 ppm free chlorine). The rate of degradation depends on two important factors 1) degradation is more rapid at high pH than at neutral or low pH, and 2) the presence of transition metals such as iron, will catalyze the oxidation of the membrane. 

The mechanism of degradation is the loss of polymer crosslinking. This results in the membrane polymer dissolving, similar to a nylon stocking when exposed to chlorine bleach. Damage is irreversible and will continue as long as the membrane is exposed to the oxidizer. 

Chloramines also pose a risk to polyamide, composite membranes (see Chapter 8.5.2.1.2). Chloramines are virtually always in equilibrium with free chlorine. Although the tolerance of the FilmTec FT30 membrane to pure chloramines is 300,000 ppm-hrs, FilmTec still recommends that influent water with chloramines be dechlorinated prior to the membrane. In most cases, ammonia is added to chlorine to generate chloramines. This leaves open the possibility that there is still some free chlorine available. The most successful chloramine applications seem to be found in wastewater systems with a resident concentration of ammonia, to which chlorine is added to make the chloramines. 

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Schroeder, H.A., Tipton, l.H. and Nason, A.P. 1972 Trace metals in man strontium and barium. Journal ofChronic Diseases 19 545-571. 

Schroeder HA, Tipton IH, Nason AP. 1972. Trace metals in man Strontium and barium. J Chronic Dis 25 491-517. 

Calcium and Barium Sucrates.—The formation of insoluble compounds of sucrose with salts of the metals of the alkaline earths, calcium, barium and strontium has long been known. The use of salts of barium and strontium commercially was proposed and tried out in Europe many years ago but failed to receive approval, perhaps largely on account of the fear of traces of the known poisonous salts in the finished product. The cost of the necessary salts and the difficulty of reconversion into usable form also had great commercial influence. The value... 

Table 37 compares the contents of trace elements in the ash of various grass and tree species from the Savanna ecosystems of East Africa. We can see that nickel, barium, and strontium accumulate in the tree organs (twigs), whereas the accumulation of other metals is pronounced in grasses. 

An interesting application of a cation exchanger has been given in relation to trace analysis of sodium-, potassium-, barium-, and strontium chlorides [147]. Metal cations are retained by the cation exchanger. Concentrated HCl run onto the column precipitates NaCl, KCl, BaCh, and SrCh, whereas trace metals present are eluted. 

A very considerable number of elements has been shown to occur in a wide range of animal tissues and fluids in such minute amounts that they can appropriately be described as traces. Numerous studies of biological materials from widely separated sources have established the fact that copper, manganese, zinc, iodine, cobalt, nickel, aluminum, chromium, tin, silicon, titanium, lead rubidium, lithium, molybdenum, arsenic, fluorine, bromine, barium, and strontium are commonly present in low concentrations in blood, milk, and tissues of higher animals, and other metals, such as silver, gold, boron, cadmium, and cerium are occasionally present. No doubt others will be detected as more-refined analytical methods are developed. 

Sulfates are widely known in the environment and a few such as those of calcium, strontium, barium, and lead are insoluble. The sulfates of magnesium and the alkali metals are soluble and so are typically found in brines and salt deposits. Sulfites are rarer because of the ease of oxidation, but calcium sulfite, perhaps as the hemihydrate, can be detected on the surface of building stone in urban atmospheres containing traces of sulfur dioxide (Gobbi et al, 1998). 

Trace metals can be divided into three categories those known to be dietary essentials, those possibly essential, and nonessentials. Metals known to be essential to higher animals include chromium, cobalt, copper, iron, manganese, molybdenum, selenium, tin, vanadium, and zinc. Several additional metals considered as possibly essential on the basis of suggestive but inconclusive evidence include arsenic, barium, cadmium, nickel, and strontium. The other metals are presently classified as nonessential. 

Total hardness of water (TH) is defined as the concentration of alkaline earth metal ions. Since beryllium and radium do not occur and strontium and barium occur only in traces the total hardness is the molar concentration of calcium and magnesium ions in mmol/L [7]. In practice, country-specific definitions are common. For example, in Germany the term Grad Deutsche Harte (°dH) is used. 1° dH is equivalent to a concentration of c(Ca -I-Mg +) = 0.178 mmol/L [1, 3]. Water is very individual in its composition depending on its origin. A lot of different dissolved filter materials affects the electrical and chemical properties of water. 

The hydrolytic ability of hydrochloric acid may be affected by salts present in the reaction mixture. The effect may be based on salting out of the hydrolysis products from the reaction mixture. For instance, magnesium and ammonium sulfates showed such an effect.117 Zinc and cadmium cations, as well as acetate and sulfate anions, decreased the hydrolysis rate with 0.1 M hydrochloric acid, whereas calcium, strontium, and barium ions exerted no effect.141 Neither copper metal, stainless steel, tin, nor sulfur dioxide impeded the hydrolysis with a solution of 0.03-0.05 M hydrochloric acid.142 There is a report143 that traces of metal salts affect the rate of hydrolysis but there are also contradicting findings144 145 that... 

Elements like aluminium, barium, cadmium, lead, strontium, and traces of others exist in different forms in plastics additives, and may enter the body through food and water as well as through air breathed in. These elements do not serve any known purpose in the body, but they are still absorbed, and as a result, the average adult body can contain significant amounts of them. Some of these resemble human elements (e.g., strontium resembles calcium closely, and a lot of it is absorbed easily in bones, to the extent that approximately 320 mg can be found in the body of an average person, which is far more than many of the essential elements, while even gold and uranium can exist in quantities of 7 and 0.07 mg, respectively), and are retained and deposited preferentially either in the skeleton (e.g., uranium, binds specifically to the phosphate of the bones) or elsewhere for example in the liver, where liver proteins can trap and deposit some of these heavy metals, like cadmium. 

Generally, however, more than 95% of ash is made up of silicon, aluminum, iron, and calcium in their oxide forms, with magnesium, potassium, sodium, and titanium representing the remaining major constituents. Ash may also contain a wide range of trace constituents in highly variable concentrations. Potential trace constituents include antimony, arsenic, barium, cadmium, chromium, lead, mercury, selenium, strontium, zinc, and other metals. 

The strontium content in the lithosphere is ca. 370 mg/kg (i.e., ppm wt.), but, owing to its chemical reactivity, the metal does not occur free in nature. The chief strontium containing minerals are the sulfate celestite or celestine [SrSO, orthorhombic] and the carbonate strontianite [SrCOj, orthorhombic], but strontium traces can also be found in calcium and barium-containing minerals. Nevertheless, strontium minerals rarely concentrate in large ore deposits, and the chief ore is only represented by celestite because there are no known economically workable strontianite deposits. However, strontium occurs widely dispersed in seawater and in igneous rocks as a minor constituent of rock-forming minerals. 

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