Seawater cation exchange

Ra is continually produced in sediments by the decay of insoluble Th parents (see Fig. 7). While in fresh water Ra is bound tightly to particle surfaces, in seawater Ra readily undergoes cation exchange with other dissolved constituents. This provides an additional source of Ra to the water column, which controls the frequently observed non-... 

Removal of inorganic interferences, particularly the removal of bromide interference in seawater by fivefold dilution of the sample, the removal of nitrate by addition of sulfamic acid, and the removal of metals by passage through Amberlite IR 120 cation exchange resin. 

Tyree and Bynum [132] described an ion chromatographic method for the determination of nitrate and phosphate in seawater. The pre-treatment comprised vigorous mixing of the sample with a silver-based cation-exchange resin, followed by filtration to remove the precipitated silver salt. 

Fukishi and Hiiro [222] determined sulfide in seawater by this technique. The method is based on the generation of hydrogen sulfide by the addition of sulfuric acid to the water sample. The gas permeated through a microporous polytetrafluoroethylene (PTFE) tube, and was collected in a sodium hydroxide solution. The carbon dioxide in the permeate was removed by adding a barium cation-exchange resin to the sodium hydroxide solution. Injection into the... 

Dehairs et al. [78] describe a method for the routine determination ofbarium in seawater using graphite furnace atomic absorption spectrometry. Barium is separated from major cations by collection on a cation exchange column. The barium is removed from this resin with nitric acid. Recoveries are greater than 99%. 

Atomic absorption spectrometry has been used to determine caesium in seawater. The method uses preliminary chromatographic separation on a strong cation exchange resin, ammonium hexcyanocobalt ferrate, followed by electrothermal atomic absorption spectrometry. The procedure is convenient, versatile, and reliable, although decomposition products from the exchanger, namely iron and cobalt, can cause interference. 

Matthews and Riley [336] determined indium in seawater at concentrations down to 1 ng/1. Preconcentration of metals on a cation exchanger was fol-... 

The classic work of Dawson and Pritchard [264] on the determination of a-amino acids in seawater uses a standard amino acid analyser modified to incorporate a fluorometric detection system. In this method the seawater samples are desalinated on cation exchange resins and concentrated prior to analysis. The output of the fluorometer is fed through a potential divider and low-pass filter to a comparison recorder. 

Sayles, F. L., and P. C. Mangelsdorf Jr. (1979), "Cation-Exchange Characteristics of Amazon River Suspended Sediment and its Reaction with Seawater", Geochim. Cosmochim. Acta 43, 767-779. 

Cation exchange is particularly important in estuaries because particles transported in river flows experience increasingly higher major cation concentrations as the freshwater mixes with seawater. This process is represented by... 

After delivery to the ocean, clay minerals react with seawater. The processes that alter the chemical composition of the terrigenous clay minerals during the first few months of exposure are termed halmyrolysis. These include (1) cation exchange, (2) fixation of ions into inaccessible sites, and (3) some isomorphic substitutions. Another important transfiarmation is flocculation of very small (colloidal-size) clay particles into larger ones. 

Most cation exchange occurs in estuaries and the coastal ocean due to the large difference in cation concentrations between river and seawater. As riverborne clay minerals enter seawater, exchangeable potassium and calcium are displaced by sodium and magnesium because the Na /K and Mg /Ca ratios are higher in seawater than in river water. Trace metals are similarly displaced. 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

The clay minerals carried by rivers into the ocean represent a net annual addition of 5.2 X 10 mEq of cation exchange capacity. Most of these exchange sites are occupied by calcivun. Within a few weeks to months following introduction into seawater, sodium, potassium, and magnesium displace most of the calcium. As shown in Table 21.7, this uptake removes a significant fraction of the river input of sodium, magnesium, and potassium. 

The ratio N(5)/N(-3) is approximately 3 1. The Fe(3)/Fe(2) ratio is 4 1. Thereby it is important to see that Fe(2) exists in form of the free cations Fe2+ or as positively charges complex FeCl+ and thus is subject to cation exchange, while Fe(3) occurring mainly in form of the zero charged complex Fe(OH)3° is not. U(6) clearly dominates compared to U(5) and U(4). In contrast to U(4), U(6) is considerably soluble and thus more mobile. But the predominant U(6) species are the negatively charged complexes (U02(C03)34, U02(C03)22"), which are subject to interactions with e.g. iron hydroxides and thus mobility may be limited. The different proportions of the reduced form of the total concentration for N, Fe, and U are in accordance with the theoretical oxidation/reduction succession (see also Fig. 20). The oxidation of Fe(2) to Fe(3) already starts at pE values of 0, the oxidation of N(-3) to N(5) only at pE= 6, while the oxidation of uranium is already finished at a pE value of 8.451, which was determined in the seawater sample. 

This approach has several drawbacks. The extractant gradually contaminates the ion-exchange resin and gradually permeates the seawater. The ion-exchange capacity of the resin is used ineffectively, because of the relatively low selectivity of Dowex 50 cation exchangers for Mg relative to Na in the presence of seawater. The equilibrium capacity of Dowex 50x8 toward Mg by sorption fix>m seawater does not exceed 1 mg-equiv/cm of the resin bed. 

In recent years, a number of processes have been developed [35-37] for the ion exchainge recovery of Mg from seawater by applying weak-acid (carboxylic) ion exchangers. The following monofunctional carboxylic acid cation exchangers (methylmethacrylate-DVB type) were... 

CEC-clay, cation exchange on estuarine clay minerals MOR, mid-ocean ridge and other seawater-basalt interactions. 

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