205
I 175
I
L-
near plant 2 cm/yr
189
~
217
downstream 3 cm/yr
Flgure 2. Electron impact mass spectrum of a compound subsequently identified as 2,6-di-fert-butylbenzoquinone 4-(carboxymethyl)methide. This mass spectrum was obtained at 70 eV on a Hewlett-Packard5982 G U M S system.
& 6
Core Depth l c m l
CH,CH&OOR
I
CH,CHEOOH
I
CHCH2COOH
II
12 18 2L 30 36 1 2 18
Core Depth ( c m l
Figure 1. Plot of the ratio of 2,6-di-tert-butylphenol to 2,6-di-tertbutylbenzoquinone(average of two replicates) vs. core depth (in cm) at two sites in the Pawtuxet River (Rhode Island); see Table I. The sedimentation rates and correlation coefficients are given.
205 was C12H1303, and that mle 217 was C15H210. Furthermore, this unknown compound and the propionic acid were formed in an independent experiment by stirring a sediment sample containing various esters of 3-(3,5-di-tert-butyl-4hydroxypheny1)propionic acid in high-purity water for 2 weeks. These data suggest that the unknown was a quinone (carboxymethy1)methideof the following structure:
OH R = CH, or C,,H,,
!
OH
0
This identification of a quinone methide as an environmental oxidation product of a phenolic compound is unique. On the basis of this finding and of the observation of other facile phenol oxidations, we suggest that the general environmental fate of phenolic antioxidants is their transformation to quinones or quinone methides. Acknowledgment The cooperation of the company officials and plant personnel in obtaining the wastewater samples is gratefully acknowledged. Literature Cited (1) “Kirk-Othmer Encyclopedia of Chemical Technology”, 3rd ed.;
Since this compound (2,6-tert-butylbenzoquinone 4-(carboxymethy1)methide) was structurally related to the propionic acid but had two fewer hydrogen atoms, it is likely that oxidation of this acid had taken place in the river water. The source of the propionic acid is probably hydrolysis of the methyl ester; the C18 ester is less likely to hydrolyze because of its strong tendency to adsorb to the river sediment (2).
Wiley: New York, 1978; Vol. 3, p 139. (2) Lopez-Avila, V.; Hites, R. A. Environ. Scz. Technol. 1980, 14, 1382. (3) Jungclaus, G. A,; Lopez-Avila, V.; Hites, R. A. Environ. Sei. Technol. 1978,12,88. 141 , , Eelinton. G.. Ed. “Environmental Chemistrv”: The Chemical Soiety: United Kingdom, 1978; Vol. 1,Special”Report No. 35. Received for reuiew February 9,1981. Accepted July 16,1981. This work has been supported by the U.S. Environmental Protection Agency (Grant No. R806350) and by the National Science Foundation (Grant No. ENV-75-13069).
Removal of Elemental Mercury from Wastewaters Using Polysulfides David M. Findlay and Ronald A. N. McLean* Research Centre, Damtar Inc., Senneville, Quebec H9X 3L7
Following the discovery in the late 1960s in Sweden that inorganic forms of mercury released in effluents from industrial plants could be converted to methylmercury in the natural environment ( I ) , a number of measures were taken to prevent the release of mercury. In North America the chloralkali industry effected a rapid reduction in its losses to the environment in the early 1970s. In the treatment of wastewater, the most common methodology involved treatment with sodium sulfide or hydrosulfide to convert the mercury 1388
Environmental Science & Technology
to mercury(I1) sulfide which could be precipitated ( 2 , 3 ) .In some plants, ion-exchange resins and activated carbon were used as polishing steps after sulfide precipitation to reduce mercury concentrations in effluents even further. In our studies (4-7) on the control of mercury losses from a chlor-alkali plant, the form of mercury in the various effluents and solid wastes was studied. I t was discovered that a considerable proportion of the mercury in the wastewaters and solids was elemental mercury (6) and that the traditional
0013-936X/81/0915-1388$01.25/0
@ 1981 American Chemical Society
The use of sulfide to precipitate mercury from water has been instrumental in reducing mercury losses, especially from chlor-alkali plants, to the environment. However, it has been discovered in this work that sulfide treatment, though excellent for the removal of ionic mercury species from industrial wastewaters, does not effect adequate removal of elemental mercury. Polysulfide salts (e.g., Na2S5) have been shown to
provide the necessary mild oxidant for the efficient removal of elemental mercury from solution without the undesirable oxidation of the sulfide in precipitated mercury(I1) sulfide. A process involving sodium polysulfide has been successfully used over a period of 4 yr to treat the wastewaters and sludges from a mercury-cell chlor-alkali plant.
sulfide treatment system (2) was not an effective method for insolubilizing this form of mercury. Elemental mercury is not oxidized under the reducing conditions which normally exist in a sulfide treating system; thus, it does not react with sulfide and precipitate as mercuric sulfide. An oxidant was clearly required in the sulfide process. The strong oxidants, chlorine, hypochlorite, and chlorate are often found in chlor-alkali plant wastewaters but, since they cause the oxidation of sulfide and the release of mercury(I1) to solution, they are'not suitable for this purpose. They should be removed from any wastewater before sulfide treatment. However, elemental sulfur and polysulfide salts will also oxidize elemental mercury to form mercuric sulfide. Polysulfide salts, which have the advantage of being water soluble and more reactive, react with mercury according to
As a precaution to prevent this reaction, it is advisable to add another metallic ion which will precipitate the excess sulfide, for example, Fe3+. The combined polysulfide plus sulfide treatment has proved to be particularly effective in chloralkali plants which have existing sulfide treatment systems as shown by the following examples. Treatment of the Effluent from the Cell Sewer in a Chlor-Alkali Plant. Samples from a chlor-alkali plant cell sewer, at pH 9, had an elemental mercury concentration of 140 yglL and a total mercury concentration of 570 pg/L. These samples were treated with sodium hydrogen sulfide and/or sodium polysulfide (NaZS,, n = 4), and 100 ppm of FeCl3. The p H of other samples was adjusted to various levels between 7 and 14, and these were then treated in the same manner. The reagent concentrations used and the resulting mercury concentrations remaining in solution are shown in Table I. The use of sodium sulfide alone removed only 59% of the elemental mercury, whereas sodium polysulfide alone removed 93% of the elemental mercury. The optimum pH for the treatment with the mixture of sodium polysulfide and sodium hydrogen sulfide is between 9 and 11,since this attains more than a 99% removal of the mercury. Above a pH of 11,the mercury concentration in solution begins to increase again because of the formation of soluble HgSZ2-, and mercury removal becomes less efficient. When the recommended treatment with sodium polysulfide (50 ppm), sodium hydrogen sulfide (50 ppm), and ferric chloride (100 ppm) was carried out on samples from the cell sewer with much more mercury in them (11ppm), the removal efficiencies at pH 9-12 were all over 99% with less than 1 ppb elemental mercury and between 20 and 40 ppb of total mercury in the final effluent. In an actual chlor-alkali plant treatment system, the simple addition of 20 ppm sodium
Hg
+ Sf12-
HgS
+ Sn-12-
+
-
= 3-6)
(1) It was found that polysulfide provides an efficient method of removing elemental mercury from wastewaters. A mixture of sodium polysulfides is prepared by saturating an alkaline solution of sodium sulfide, or sodium hydrosulfide, with sulfur. However, since both elemental and ionic mercury are present in the contaminated media, it is preferable to use an excess of sulfide so that not all of the sulfide is converted to polysulfides. Polysulfide will also precipitate ionic mercury; however, a greater amount of sulfur as sodium polysulfide is required to precipitate the same amount of mercuric ion. In the treatment of mercury-containing wastewaters with sulfides, it is necessary to prevent the addition of excess sulfide in solution since the precipitated mercury sulfide can be redissolved to form the soluble mercuric disulfide complex according to HgS S2- HgS2'(2) +
(TZ
Table 1. Treatment of Mercury-Contaminated Wastes with Sulfides and Polysulfides sample
(1) cell sewer A (collected samples)
treatment a
(a)50 ppm NaHS
9
140
570
58
(b) 50 ppm Napsn (c) a b (d) a b (e) a 4-b 50 ppm Na2S, 50 ppm NaHS 37 ppm 20 ppm Na2S, NaHS 75 ppm FeCI3 (a) 50 ppm NaHS
9 7 9-1 1
140 140
570 570
10 10 1-2 94 1
+ +
(2) cell sewer B (collected samples) (3) cell sewer C (actual system)
(4) brine sludge A
+
+ +
(b) 50 ppm Napsn (c) a b 50 ppm Na2S, (a) 50 ppm NaHS (b) 50 ppm NaHS 50 ppm Na2%
+
(5) brine sludge B (6) perimeter sewer
E
PH
mercury concn, @g/L pretreatment posttreatment elemental total elemental total
All include 100 pprn FeCI3 unless otherwise noted.
+
13 9-12 10 10.5 10.5 10.5 10.5 9.5 9.5
140
570
140 NDb
570 1100
70
a7
1
150 150 150 720 110 110
5200
30 3 3 1 76 3
5200 5200 1390 250 250
94 10 10 1-4 270 20-40
efficlency of treatment, % elemental total
59
a4
93 93
98 98
99+ 97
99+ 53 99+
11
99
87
430 65 53 20-30 110 20
80 98 98 99+ 31 97
92 99 99 99 60 92
ND = not determined.
Volume 15, Number 11, November 1981
1389
polysulfide to the existing treatment (consisting of the addition of 37 ppm sodium hydrosulfide and 75 ppm ferric chloride) had a dramatic effect on the concentration of mercury in the system. The average mercury concentration in the treated wastewater in the 2 weeks prior to the use of polysulfide was 87 ppb, and after the use of polysulfide, with no other change in the system, the concentration dropped to an average of 11 ppb. Treatment of the Brine Sludge from a Chlor-Alkali Plant. One of the major sources of mercury losses from a mercury-cell chlor-alkali plant is in the alkaline treatment of the recycled electrolyzed brine solution to effect the removal of calcium, magnesium, and some transition metals before the return of the brine to the cells for electrolysis. The alkali used in this treatment usually comes directly from the sodium hydroxide made in the process, and this sodium hydroxide often contains high concentrations of elemental mercury. Thus, the sludge which results from the brine treatment contains elemental mercury. Samples of brine sludges from chlor-alkali plant processes were treated as described in Table I and then filtered. In typical brine sludge samples (Table I, no. 4) the elemental mercury concentration was 150 ppb of a total of 5200 ppb. Treatment with 50 ppm sodium hydrogen sulfide alone reduced the elemental mercury concentration t o 30 ppb and the total mercury concentration to 430 ppb. Treatment with sodium polysulfide alone reduced the elemental mercury to less than 3 ppb of a total of 65 ppb, and treatment with both sodium polysulfide and sodium hydrogen sulfide reduced the elemental mercury concentration to less than 3 ppb and the total mercury concentration to 53 ppb. A similar improvement in the treatment efficiency was observed when polysulfide was used to treat perimeter sewer samples. Clearly, the sodium polysulfide not only aids in the removal of elemental mercury from the filtrate but also allows a greater efficiency of removal of other forms of mercury than sodium hydrogen sulfide. In samples with high concentrations of elemental mercury (Table I, no. 5), treatment with polysulfide alone reduced the elemental mercury to less than 1ppb, and the total mercury to 20-30 ppb. The treatment of mercury-contaminated wastewaters from a chlor-alkali plant has now been used for a period of 4 yr. The treatment has been successful in reducing mercury losses
1390
Environmental Science & Technology
considerably. However, the following precautions should be noted: (1) The pH of the wastewaters must be maintained between 9 and 12. ( 2 ) The solution of sodium polysulfide, prepared by dissolving 2.0 kg of sulfur in 10.0 L of water containing 0.5 kg of sodium sulfide and 0.7 kg of sodium hydroxide should be made up once a week. (3) If chlorine or hypochlorite are present in the wastewater, these should be removed before or coincidently with, polysulfide addition by the addition of a suitable reducing agent, such as sodium sulfite. (4)Ferric chloride (75-100) ppm should be added to remove excess sulfide and to assist in flocculation of mercuric sulfide. In addition to the treatment of wastewaters, polysulfide solutions have been used to convert mercury to mercury sulfide in chlor-alkali plant solid waste disposal sites to prevent the release of elemental mercury in the gas phase and in runoff to waterways. Polysulfide treatment has also been suggested to reduce gas-phase emission of elemental mercury from mine surfaces ( 8 ) . Acknowledgment
We thank Ms. M. 0. Farkas for aid with the mercury determinations, A. Gaulin for supervising the plant trials, and Dr. E. J. Tarlton, Director, Domtar Research, for approving publication of this paper. Literature Cited (1) Jensen, S.; Jernelov, A. Nature (London) 1969,223, ‘753. (2) Perry, R. “Mercury Recovery from Contaminated Waste Water and Sludges”,U.S. EPA Report 660/2-‘74-086, 1974. (3) Habashi, F. Enuiron. Sci. Technol. 1978,12, 1372. (4) Brouzes, R. J. P.; McLean, R. A. N.; Tomlinson, G. H. “The Link Between pH of Natural Waters and the Mercury Content of Fish”, paper presented a t the U S . National Academy of Sciences, National Research Council Panel on Mercury, Washington, 1977. ( 5 ) Brown. J. R.: Bancroft. G. M.: Fvfe. W. S.: McLean. R. A. N.: Enuiron,’ Sci. Technol. 1979,13,’114214. (6) McLean, R. A. N.: Farkas. M. 0.:Findlav, D. M. “Polluted Rain”; Toribara, T. Y., Miller, M. W., Morrow, P. E., Eds.; Plenum Press; New York, 1980; pp 151-73. (7) , , Tomlinson. G. H.: Brouzes. R. J. P.; McLean. R. A. N.; Kadlecek, J. “Ecological Impact of Acid Precipitation”; Drablos, D., Tollan, A,, Eds.; SNSF; Oslo, 1980; pp 134-7. (8) Randall, M. Canadian Patent 416 629, 1943. I
,
Receiced for review March 6,1981. Accepted July 22,1981.
