Oxygenation of Ferrous Iron(2)

T h e ferrous iron solutions were freshly prepared by dissolving Fe(C104)2 (G. F. Smith Chemical Co.) in 0.1M HC104. Bicarbonate-COz buffers\vere prepared from NayCOzsolution by bubbling a n airCOZ mixture through the solution. Various pH values\vere obtained by using different air-CO? mixtures. These air-COZ compressed gas mixtures\\'ere prepared and analyzed by General Dynamics Corp., Eo;ron. hIass. The test solutions were\vel1 thermostated (*0.2' C.) and we

re vigorously stirred during the run. All pH measurements were made with a Beckman Model G pH meter. During any one run the pH was kept constant within 0.04 pH units. To a 1.5-liter borosilicate glass beaker was added a N a 2 C 0 3 solution. after temperature equilibrium\vas attained, a n air-CO1 mixture was bubbled through this stirred solution. I\'hen the pH of the solution had reached the equilibrium value for the particular partial pressure of COz in the gas mixture being used, the ferrous iron solution was added and a stopwatch was started. T h e volume of test solution a t this point was 1.O liter. At appropriate time intervals, aliquots of the test solution were withdrawn and placed in sulfuric acid solution to quench the reaction. Colloidal ferric hydroxide was dissolved by boiling this acid solution. The ferrous iron content of each aliquot was determined colorimetrically after neutralization with sodium acetate, using a Beckmann DC' spectrophotometer.Results

Stoichiometry. T h e reaction of Fe(I1) with oxygen generally leads directly IO ferric oxides or hydroxides. T h e stoichiometric relationship beingFe(l1)

+ 1/401 f 20H-+1/2Hz0-+

Fe(0H)a (s)

(5)

Effect of O x y g e n a t i o n o f Ferrous I r o n o n Surface Waters Prime examples are the rivers draining the coal mining areas of Pennsylvania, Ohio, and West Virginia. As a result of acid mine drainage, these waters contain large quantities of iron and have low pH values. Fifty per cent of the acidity in acid mine drainage arises from the oxygenation of ferrous iron; the remainder arises from the oxygenation of sulfide or polysulfide. In many cases, the drainage from these mines contains appreciable quantities of ferrous iron which, upon entering a stream, reacts with the dissolved oxygen present to form unsightly ferric oxide deposits. The accompanying reduction in pH may inhibit the selfpurification of the river b y providing an unfavorable environment for microorganisms. Water treatment operators who derive their raw water from reservoirs are also well aware of the common occvrrence of high iron concentration in the hypolimnetic zones during periods of stagnation and stratification of the reservoir. Water supplies derived from ground waters that have passed through iron bearing strata frequently contain appreciable quantities of ferrous iron which, upon exposure to the atmosphere, are oxidized to insoluble ferric oxide.

The oxidation may occur under certain conditions-especiallk in buffer solutions with p H values greater than 6in a stepwise fashion over the mixed iron( 11)-(111) hydroxides and magnetite (7, 8 ) . T h e ferrous iron in these intermediates is only very slowly oxidized, if at all (8). I t was experimentally ascertained that under the condition of the kinetic investigation reported here, the oxygenation of ferrous iron was in agreement with Equation 5. These measurements were made by quantitatively determining the oxygen uptake with a Warburg manometer appa

ratus. Oxygenation Rates. Several representative reaction runs at a constant partial pressure of oxygen and at various pH values are plotted in Figure 1. T h e rate of oxidation\vas first order with respect to Fe(I1) and independent of the Fe(II1) concentration. This is evident from the time dependence of any one reaction (linear relationship in

144

INDUSTRIAL AND ENGINEERING CHEMISTRY

亚铁的氧化

FERROUS I R O N O X Y G E N A T I O Nsemilogarithmic plot of Figure 1). This first order dependence with respect to Fe(I1) of the reaction rate was also confirmed by experimental runs in which the initial Fe(I1) concentration was ividely varied. Incipient addition of ferric iron up to concentrations of 10-4AW did not alter the reaction rate. The oxygen concentration in the reaction solution was kept in excess and constant during each run. Since differe n t investigators agree with respect to the dependence of the rate of reaction on the oxygen concentration, this dependence\vas not investigated in a rigorous manner. The partial pressure of oxygen\vas varied only between 76 and 155 mm. Hoii-ever. the results support the finding of others, that the rate of oxygenation of ferrous iron is dependent on the partial pressure of oxygen to the first power. Thus the rate law accounts for theTable 0. Dependence of Rate on Partial Pressure of Oxygen and (OH-)

Alkalinity 2.9 to 3.9 X 10-2 equiv. per liter. Temp. 20.5' C .kd

3 1 h - l A t r n . -1

Po2 dtm. 0.107 0.107 0.153 0.153 0.153 0.174 0.184 0.184 0.195 0.1951

(OH -1"

x

k,b

105

x

.\Iin. -110'

x

k'C 102

Liter2 3Iole-2

x

10-13

1.86 2.51 3.16 4.56 5.02 7.25 9.12 12.0 18.2 22.4

0.063 0.090 0.233 0.478 0.492 1.71 2.44 4.65 9.13 17.8

0.589 0.841 1.53 3.13 3.21 9.82 13.2 25.3 46.8 91.4

1.70 1.33 1.53 1.50 1.28 1.87 1.59 1.76 1.41 1.83

Computed froni measured pH ( K u= (1.7 X 10 1 4 1 . 2.3 k,= .lope of firbt order plots -din[Fe(II)I k,= k,'Pn,. k= k'/(OH-)*. (Figure 1): k,=di'

change in Fe(I1) concentration during the course of the reaction a t a constant pH value as a function of Fe(I1) concentration and partial pressure of oxygen, PO2. The change in rhe rate of reaction with p H as shoivn in Figure 1 can be explained by the influence of hydroxyl ions on the reaction rate. The logarithm of the .: rate constants X, (k,= - d l n[Fe(II)] J'dt) divided by the partial …… 此处隐藏：7078字，全部文档内容请下载后查看。喜欢就下载吧 ……