Coprecipitation of Mercury(II) with Iron( 111) Hydroxide Yoshikazu lnoue and Makoto Munemori* Laboratory of Environmental Chemistry, College of Engineering, University of Osaka Prefecture, Mozu-Umemachi, Sakai, 59 1, Japan
Coprecipitation of mercury(I1) with iron(II1) hydroxide as a function of pH in the range of p H 4 to 1 2 was studied in the presence and absence of halogenide ions except iodide ion. Mercury(I1) is coprecipitated with iron(II1) hydroxide in the whole range of pH studied. Fluoride ion does not affect the coprecipitation. Chloride and bromide ions suppress the coprecipitation a t lower pH values, depending on the stability of mercury(11)-halogenide complexes and the concentration of these halogenide ions. The mercury(I1) species coprecipitated was inferred to be Hg(OH)2from the chemical equilibrium considerations. Mercury is used in paints, paper, and electrical apparatus and in many industrial operations, especially in the electrolytic production of chlorine and sodium hydroxide. These industrial users o f mercury cannot recover all of the mercury that is used duriing production. For example, in the process of electrolysis of sodium chloride, about 1 kg of mercury will be lost from the process for every 900 kg of chlorine that is produced ( I ). Effluents released from these factories have artificially polluted the natural waters. Mercury may be present as various chemical forms in natural waters, and partially concentrated a t the bottom as sediments after interaction with colloids and suspended matter. Inorganic mercury concentrated a t the bottom may be changed to organic mercury by the action of organisms (2-4). Mercury has been internationally noted as a pollutant, because organic mercury, especially alkylmercury, has a toxic effect on man through the food chain. I t is known that heavy metal oxides play an important role in the concentral ion of other heavy metals. In natural waters, adsorption on h,ydrous iron(111) and manganese(1V) oxides is recognized as the important mechanism in controlling the concentration of heavy metals (5-7). It has been reported that mercury(I1) is adsorbed by oxides such as a-SiOz, a-FeOOH, n-AlzO3, and 6-NInOz and coprecipitated with lanthanum(II1) hydroxide (8, 9). The present investigation was undertaken to elucidate the coprecipitation behavior of mercury(l1) with iron(II1) hydroxide in the presence or absence of halogenide ion. Although the concentrations of iron(II1) hydroxide and of mercury(I1) are higher than normally would be encountered in an aquatic environment, the results should establish a guideline for future environmental research. In the present paper, we describe the coprecipitation study of the mercury(I1)-iron(II1) system and discuss the mercury species which is responsible for coprecipitation from the viewpoint of chemical equilibrium. Experimental
Reagents. Mercury(I1) stock solution, 5 X lo-' M, was prepared by dissolving mercury(11) sulfate in dilute sulfuric acid. Iron(II1) stock solution was prepared by dissolving iron(II1) sulfate in dilute sulfuric acid and standardized by titration with potassium permanganate after reduction with zinc. Working solutions were prepared fresh daily by diluting the stock solutions. All chemicals were of analytical reagent grade. Procedure. Unless otherwise stated, a 200-mL portion of the solution containing 5 X lo-* M mercury(I1) and 7 X 10-3 0013-936X/79/0913-0443$01.00/0
M iron(II1) was used in obtaining the data reported be )w. The solution was adjusted to an appropriate pH value with sodium hydroxide and allowed to stand while it was stirred a t 25 "C. After equilibrium had been reached (it required about 10 min), the solution was filtered through a filter paper (Toyo filter paper no. 2). All p H values reported are equilibrium values. Mercury(I1) remaining in the filtrate was determined by a cold-vapor atomic absorption technique. Sodium borohydride was used as a reductant, since it can reduce mercury(I1) to mercury(0) even in the presence of halogenide ions (10). Chemical equilibrium calculations were made using the available data shown in Table I. R e s u l t s a n d Discussion
Coprecipitation of Mercury(I1) with Iron(II1) Hydroxide. Mercury sulfate was used throughout the present study as the source of mercury(I1) ion. The effect of sulfate ion on the hydrolysis of mercury(I1) was first examined. Chemical equilibrium considerations indicate that mercury is precipitated as HgO above pH 3 when its concentration is higher than 2.34 X lo-* M and that sulfate ion does not prevent the precipitation, because the formation constant of HgS04(aq) (10l3) is very small compared to that of HgO ( 102j.7).In reality, however, no precipitate appeared when solutions containing 5 X lo-* M mercury(I1) and 2 X M sulfate were allowed to stand for about 70 min a t 25 "C after adjusting the pH to 6 and 10. I t seems that sulfate ion interferes kinetically with the formation of HgO. In the presence of iron(III), however, mercury(I1) was precipitated in a wide range of pH as shown in Figure 1. The concentration of sulfate in the range from 5 X lo-* to 1 X M did not affect the result, suggesting the absence of the effect of ionic strength on the precipitation. The concentration of mercury(I1) remaining in aqueous phase at pH 8 was 2.4 X 21,which is far less than that expected from the solubility of HgO (2.34 X M, i.e., 45 ppm of Hg). These results indicate that mercury(I1) is coprecipitated with iron(II1) hydroxide. Effects of Halogenide Ions on Coprecipitation of Mercury(I1). As halogenide ions are known to form stable complexes with mercury(II), their effects on the coprecipitation of mercury(I1) with iron(II1) hydroxide were studied a t various pH values. Fluoride Ion. At pH above 5, fluoride ion a t a concentration of 5 X M did not affect the coprecipitation of mercury(I1) with iron(II1) hydroxide. This is consistent with chemical equilibrium considerations: the formation constant of the niercury(I1)-fluoride complex is so small that fluoride ion does not affect the concentration-pH diagram for mercury(I1) species (Figure 2). A predominant species of mercury(I1) in the presence and absence of fluoride is Hg(OH)2, as shown in Figure 2, and hence the mercury(I1) species which is coprecipitated with iron(II1) hydroxide is assumed to be Hg(OH)2. At pH 4, the extent of coprecipitation of mercury(I1) somewhat decreased. At this pH, the complex formation between iron(II1) and fluoride becomes appreciable and, therefore, fluoride ion may interfere with the coprecipitation of mercury(I1). Chloride Ion. Figure 3 shows the effect of chloride ion on the coprecipitation of mercury(I1) with iron(II1) hydroxide.
@ 1979 American Chemical Society
Volume 13, Number 4, April 1979
443
Table I. Cumulative Stability Constants ( P n )of Some Mercury(l1) and Iron(ll1) Complexes ( 77, 72) mercury(l1) complexes log P2 log 83
log 81
Ion
F-
1.01 6.7 9.1 12.9 1.3
GI-
BrIso42-
c
1.03 13.2 17.4 23.9 2.4
IO
0
6
1.05 14.2 19.8 27.7
12
PH
Figure 1. Coprecipitation of mercury(l1) with iron(ll1) hydroxide at'various pH values
lW
log P4
PI
Iron(ll1) complexes log P2
log P3
9.2 2.2
11.9 1.2
5.2 1.5 0.6 1.3 4.2
15.2 21.1 29.9
5.5
n
" i
Lg", Hi%.
0
4
,
8
6
10
1
12
PH Figure 4. Distribution diagrams for mercury(l1)-chloride complexes as
a function of pH at various concentrations of chloride. The species
6
4
8 PH
Eauilibrium
Rrac tions
~ g '+ ~H ~ O = HEOH+ HgOH'
HgO(s)
-
+ H20 * H,O
+
= Hg(0H)3-
12 constants
10-3 .
H+
+ H20 = Hg(OH)z
Hg(OH)z
10
HgOHCl which has been reported in chloride solution ( 74) is not considered in this figure, because no value for the equilibrium constant for HgOHCl(s) = HgOHCi(aq) is available: (A) 1 X M,(6) 5X M, and (C) 5 X M
10-2.h
H'
lo-ll.;
+ H-
lo-3.7
= Hg(OH12
Figure 2. Concentration-pH diagram for mercury(l1)-hydroxide complexes at 25 OC. The diagram was calculated using representative values ( 73)for the equilibrium constants for solubility and hydrolysis 0
1001
4
1
'
6
"
'
'
a
'
10
12
PH
Figure 5. Effect of bromide ion on coprecipitation of mercury(l1) with iron(ll1) hydroxide at various pH values. Concentrations of bromide: (A) M, and (C) 5 X lod2 M 1X M,(B) 5 X
O;
6
"
0
"
'
10 '
'
12 .
PH
Figure 3. Effect of chloride ion on coprecipitation of mercury(l1) with iron(li1) hydroxide at various pH values. Concentrations of chloride: (A) 1X M,(B) 5 X M,and (C) 5 X M
In the presence of chloride ion the coprecipitation of mercury(I1) is prevented a t lower pH values. With the increase in chloride ion concentration, the coprecipitation occurs a t higher pH values. The effect of chloride ion will be ascribable 444
Environmental Science & Technology
to the complex formation between mercury(I1) and chloride ions. Distribution diagrams for mercury(I1)-chloride complexes a t chloride ion concentrations of this study are shown in Figure 4. At lower pH values, HgC12 is predominant, but a t higher pH values Hg(OH)2 becomes predominant. The formation curves for Hg(OH)2 as a function of pH correspond well to the coprecipitation curves for mercury(I1) as a function of pH (Figure 3) a t respective concentrations of chloride ion. From this correspondence, it seems reasonable to infer that mercury(I1) is coprecipitated with iron(II1) hydroxide as Hg(OH)z. Bromide Ion. The effect of bromide ion on the coprecipitation of mercury(I1) with iron(II1) hydroxide a t various pH
._ ”
1.0 I
I 0
”
6
”
IO
8
”
12
PH
Figure 7. Coprecipitation of mercury(l1) with iron(ll1) hydroxide as a function of pH: (0) in the presence of 5.4 X lo-’ M chloride ion alone; (A)in the presence of 5.4 X lo-’ M chloride, 8.1 X M bromide, M fluoride, and 3.2 X lo-’ M iodide ions 5.3 X
-
X
H”Rr_.
8 10 12 PH Figure 6. Distribution diagrams for mercury(l1)-bromide complexes as a function of pH at visrious concentrations of bromide: (A) 1 X M, M, and (C) 5 X lo-* M (B) 5 X 4
6
values is shown in Figure 5 . The pH range in which coprecipitation of mercury(I1) occurs is shifted to higher pH values with the increase in bromide ion concentration. The effect is more pronounced than in the case of chloride ion. This is due to the difference in the formation constants of mercury(I1) complexes between bromide and chloride ions, and can be explained by consulting Figures 4 and 6. From chemical equilibrium calculations, the results of which are illustrated in Figure 6, it is expected that the formation of Hg(OH)2 is suppressed by the presence of bromide ion and shifted to higher pH values depending upon the concentration of bromide ion. The shift is more pronounced in the case of bromide ion than in the case of chloride ion at corresponding concentration levels. The pH range in which Hg(OH)2 is formed corresponds well with that for the coprecipitation of mercury (Figure 5). The correspondence confirms the previous assumption that mercury(I1) is coprecipitated with iron(II1) hydroxide as Hg(OH)2. Iodide Ion. In the presence of iodide, the coprecipitation reaction is complicated, because iodide ion reacts with iron(111) to give iodine and iron(I1) in an acid medium when iron(II1) is added to the solution containing iodide ( 1 5 ) and, therefore, further study was not carried out. Mixture of Hsllogenide Ions. Coprecipitation behavior was studied in a mixture whose composition was similar to that of seawater with respect to halogenide ions, i.e., 5.4 X 10-l M chloride, 8.1 X 1.0-4 M bromide, 5.3 X loV5M fluoride, and 3.2 X 10-7 M iodide ions. The result is shown as a function of pH in Figure 7 . On the other hand, the coprecipitation was examined in the presence of 5.4 X 10-1 M chloride ion alone and the result is also plotted in Figure 7 . From these rgsults, it may be said that if coprecipitation of mercury(I1) with iron(II1) hydroxide takes place in seawater, the coprecipitation behavior would be controlled by the presence of chloride ion.
Seawater contains other anions such as sulfate, carbonate, and borate ions, besides metal ions. However, these anions react with neither mercury(I1) nor iron(II1) and, therefore, their effects on the coprecipitation may be neglected. Conclusion The results presented in this paper indicate that the formation of Hg(OH)2 is a crucial step in the coprecipitation of mercury(I1) with iron(II1) hydroxide. The formation of Hg(OH)2 is strongly dependent on the kind and concentration of halogenide ion. In the presence of an appreciable amount of chloride ion, coprecipitation of mercury(I1) occurs only in an alkaline medium. Accordingly, in seawater which contains a large amount of chloride ion (about 0.54 M) it is unlikely that mercury( 11) is coprecipitated with iron(II1) hydroxide, whereas in freshwater which contains a small amount of M) mercury(I1) is likely coprecipitated chloride ion (
