Iron oxidized sediments, proportion

The largest proportion of the total Fe was removed by ammonium oxalate, which attacks the amorphic fraction of iron oxide in the sediments (23). Among the low pH extractants, hydroxylamine was the least efficient in extracting Fe. The difference between the extraction of Fe by acid and extraction by hydroxylamine was related to the crystallinity of the hydrous iron oxide. As pure iron oxides aged (and crystallized) in the laboratory, Fe solubility in hydroxylamine declined relative to solubility in acetic acid (Table III). In San Francisco Bay sediments, the ratio of hydroxylamine-soluble Fe to acetic acid-soluble Fe increased during the period of maximum runoff to the estuary (27) suggesting the proportion of the Fe in the sediments that was freshly precipitated varied seasonally. This was expected, since periods of heavy runoff are also times of maximum Fe movement from the watershed to the tributaries of the estuary ( ). 

Carbonate iron sediments are formed when iron is precipitated in the presence of dissolved carbonic acid or as a result of interaction of the primary sediments with organic matter in the course of diagenesis. The determination of the stability of primary iron carbonates with respect to iron oxides and hydroxides was made on the basis of using the functional dependence of and on pH, calculated by the method of Garrels and Christ (1968) for the system carbonate-water. According to this dependence, in conditions of FeCOj in equilibrium with water the value of partial pressure of carbon dioxide decreases in proportion to increasing pH. 

Thus the geochemical barrier which causes nearly total precipitation of ferric oxide from water is a change in the reaction of the environment from highly acid (pH = 0-2) to moderately acid (pH = 2-4). In the case where equilibrium is reached the amount of iron hydroxide precipitated from a saturated solution is inversely proportional to the pH, the dependence being nonlinear and the main mass of sediment being deposited from highly acid solutions. 

A more widespread process consists of the hydrogenous formation of clay at the sediment-seawater interface, in deep-sea environments characterized by water depths > 4000 m, insignificant terrigenous supply, and very low sedimentation rate (South Pacific basins. The sediments mostly consist of reddish-brown oozes rich in Fe and Mn oxides (i.e., deep sea red clay ). There iron-rich smectites of the nontronite group may form in significant proportions, probably due to long-term low temperature interactions between (1) metal... 

Retention of added zinc by sediment solids was 56-60% at pH 5.0, >97% at pH 6.5, and essentially 100% at pH 8.0. Most of the zinc was present in the exchangeable and reducible fractions. Only a small proportion of added zinc was associated with insoluble organic material, as evidenced by the low recovery in DTPA and residual organic fractions (Table 12.3). The reducible phase is believed to consist of Zn strongly adsorbed to or coprecipitated with oxides and hydroxides of iron... 

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