14 A intergrade clay. Kaolinite decreased gradually from 60-30%, illite increased from 5-25%, and chlorite plus 14 A intergrade clay averaged about 40% along the length of the estuary. No relationship between the clay mineral composition and phosphorus content of the sediments could be established. The amounts of iron extracted by the acid and oxalate procedure were similar, with the oxalate values generally 1 5 2 0 % higher than the acid values. McKeague and Day (1966) have shown t h a t oxalate extracts amorphous forms of iron almost completely but crystalline oxides only slightly. Therefore, it seems likely that both the oxalate and acid extract a n amorphous form of iron and that the oxalate extracts the iron slightly better than the acid. Shulka et al. (1971) and Williams et al. (1971) studied relationships between inorganic phosphate and iron in sediments from 14 Wisconsin lakes. They found a strong correlation between the amounts of inorganic phosphate and oxalate extractable iron. The iron and phosphorus data from this study were examined to determine if a similar correlation might exist with estuarine sediments. With d a t a for the available phosphorus and extractable iron, linear regression analyses were performed using all sediment samples. Available P exhibited correlation coefficients of 0.772 with acid-extractable iron and 0.862 with oxalate-extractable iron. Next, the sediment samples were divided into two groups. Group A included samples 1-9 (average salinity of the overlying water less than 1 ppt). Group B consisted of samples 10-23 (average salinity of the overlying water above 1 p p t and increasing). A linear regression analysis was then performed for groups A and B samples. The results of this analysis appear in Figure 5 and indicate a correlation coefficient of 0.988 for group A samples and 0.863 for group B samples. A similar analysis was performed using values for the available phosphorus apd the acid-extractable iron (Figure 6). The correlation coefficients obtained were 0.981 for group A samples and 0.771 for group B samples. The high correlation coefficients obtained for the group A samples suggest that the phosphorus may be held in an “Fe-inorganic P complex” as suggested by Williams et al. (1971). The decrease in correlation for the group B samples may be due to the increasing salinity of the water which would favor desorption of phosphorus or breakdown of the Feinorganic P complex. The results of this research are generally consistent with a transport system proposed by Carritt and Goodgal (1954). In the Tar River, low salinity and high turbidity
could favor the formation of a phosphorus-solids complex. The settling of these suspended solids would result in sediments with a high phosphorus and iron content such as those observed in the upper portion of the estuary. As the water and suspended solids move further down the estuary there is a n increase in salinity and a decrease in stream velocity. These conditions would favor the coagulation and settling of suspended solids and the release of the phosphorus to solution. The suspended solids that settle under these conditions would contain less phosphorus; this was observed in the middle and lower portions of the estuary.
Literature Cited American Public Health Association, “Standard Methods for the Examination of Water and Wastewater,” APHA, WPCF, AWWA, 527-530, 13thed., 1971. Carritt, D. E., Goodgal, S., “Sorption Reactions and Some Ecological Impactions.” Deep-sea Res., 1,224-43, (1954). Hobbie, J. E., “Hydrography of the Pamlico River Estuary, N.C.,“ Rept. No. 39, Water Resources Research Institute, University of North Carolina, 69 pp, 197Oa. Hobbie, J . E., “Phosphorus Concentrations in the Pamlico River Estuary of North Carolina,” Rept. KO.33, ibid., 47 pp, 1970b. Hobbie, J. E., Copeland, B. J., Harrison, W. G., “Nutrients in the Pamlico River Estuary, N.C., 1969-1972,” Rept. No. 76, ibid.,242 pp, 1972. McKeague, J. A., Day, J. H., Canad. J . Soil Sci., 46, 13-22, 1966. Olsen, S.R., Dean, L. A,, “Methods of Soil Analysis, Part 2,” C. A. Black, Ed., pp 1035-49, Amer. SOC.Agron., Madison, Wis., 1965. Porcella, D. B., Kumagai, J. S., Middlebrooks, E. J., Proc. Amer. Soc. Cicil Eng., J . Sanit. Eng. Dic., Vol. 96, No. SA4, 911-26, 1970. Saunders, W. M., N e u Zealand J. Agr. Res., 8,30-57 (1965). Shulka, S.S., Syers, J. K., Williams, J. D. H., Armstrong, D. E., Harris, R. F., Soil Sci. Soc. Amer. Proc., 35, 244-9 (1971). Upchurch, J. B., “Sedimentory Phosphorus in the Pamlico Estuary of North Carolina,” Sea Grant Publication No. UNC-SG72-03, Univ. N. C. Sea Grant Program, Chapel Hill, K.C., 1972. Water Resources Data for ,Vorth Carolina, Part 2, Water Quality Records, 1965-69, U.S. Dept. Int., Geolog. Survey. Wentz, D. A,, Lee, G. F., Enuiron. Sci. Technol., 3, 750-4 (1969).
Williams, J. D. H. Syers, J. K., Shulka, S. S., Harris, R. F., Armstrong, D. E., ibid.,5, 1113-20 (1971). Receiced f o r reuiew April 13, 1973. Accepted September 17, 1973. This research uws supported in part by the Office of Sea Grant. National Science Foundation. and the Department of Administration, State of North Carolina. Paper ccas presented in part at the Symposium on Trace Metal and Phosphorus Interactions with Sediments, Dicision of Water, Air and Waste Chemistp, 164th National Meeting. American Chemical Society. Neu York, N.Y.. 1972.
Coagulation in Estuaries James K . Edzwald,’ Joseph B. Upchurch,’ and Charles R. O’Melia Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, N.C. 2751 4
Considerable attention has been devoted to the hydrodynamic and physical factors affecting the deposition of suspended materials in estuaries (e.g., Ippen, 1966; Krone. 1962; Partheniades, 1965; Etter et al., 1968), but little research has been concerned with the role of chemical parameters. In this paper the aggregation of colloidal Present address, Department of Civil Engineering. I-niversity of Missouri. Columbia. Mo. 65201. To whom correspondence
should be addressed. Present address, International Paper Co.. Moss Point. Miss. 39563. 58
Environmental Science & Technology
suspensions in estuaries is examined with the purpose of elucidating the chemistry of this process. Estuarine sediment, both as bed material and suspended load, can be divided into two classifications: cohesionless or coarse particles such as sand and cohesive or fine particles such as silt and clays. The deposition of cohesionless particles in estuaries depends primarily on the hydrodynamics of the system since these particles remain as individual particles regardless of the flow conditions and the solution chemistry. On the other hand, cohesive particles range in size from a small fraction of one micron to several microns. Normally a large proportion are colloi-
w The stability of clay suspensions as a function of ionic strength was determined from observations of coagulation rates. T h e stability value, a , depends on t h e type of clay mineral and on chemical solution parameters such as salinity and p H . These studies indicated t h a t clays can be destabilized in estuaries by compression of the electrical double layer. Measurements of the composition and stability of sediments collected along the 35-mile length of
the Pamlico Estuary indicated that the sediments in the upper end of the estuary were less stable than those collected in the downstream brackish areas. Kaolinite, a relatively unstable clay, was predominant in the upstream sediments while illite, a more stable clay, accumulated in the sediments near the mouth of the estuary. These observations are consistent with the view t h a t sediment deposition in t h e estuary is influenced by coagulation.
dal-i.e., particles with large surface area per unit massso t h a t t h e effects of the surface and interparticle physicochemical forces are a t least as important as those resulting from the hydrodynamics. A large fraction of these colloidal particles are clay minerals. Clays carry a n electrical charge, usually negative in natural waters, which influences their behavior in suspension. In waters of very low salinity-fresh waters-repulsive forces between the negatively charged particles dominate, resulting in stable suspensions-i.e., suspensions with little tendency for particle aggregation. Some interparticle physicochemical forces between particles are a t tractive-London-van der Waals forces. In waters that are even slightly saline the net interparticle forces become a t tractive, and particles t h a t collide tend to cling to each other to form agglomerations called flocs, whose size and settling velocity may be several orders of magnitude larger t h a n those of the individual particles. The significance of salt water to coagulation will be illustrated in this paper; consequently, the use of the term "estuary" implies a body of water in which there is a salinity transition from fresh water to seawater and excludes the tidal section of the river above the estuary. This paper is divided into three sections. First, results are presented of coagulation rate studies conducted in the laboratory using three different clays. Second, the distribution of clays in the sediments of the Pamlico River Estuary is examined with respect to coagulation kinetics. Finally. a discussion of t h e laboratory and Pamlico sections is presented.
Ward's Katural Science Establishment were used in the coagulation rate studies. Stock clay suspensions were prepared with a narrow particle size distribution using a twostep centrifugation procedure (Edzwald, 1972). A microscopic check of the size distribution of several clay suspensions indicated 80% of the clay particles had a n equivalent diameter between 1.5 and 6 p m with a mean of 4.2 p m . These clay suspensions were then used in the coagulation kinetic experiments. In this research. two types of destabilizing solutions were used a t various ionic strengths: buffered NaCl solutions a n d synthetic estuarine solutions. These solutions were buffered with S a H C 0 3 yielding clay suspensions with a p H in the range of 7.8 to 8.2. The contents of the buffered KaC1 solutions are indicated in Table I. It should be noted that the NaCl solutions contain only monovalent cations. The composition of seawater according to Lyman a n d Fleming (1540) was used as a basis for the preparation of t h e synthetic estuarine solutions (Table 11). A mathematical model describing the aggregation of particles where collisions are due to fluid motion (orthokinetic flocculation) was developed by Smoluchowski (1917):
Coagula tion O'Melia (1572) has described coagulation as a two-step process: particle destabilization and particle transport. T h e destabilization step is concerned with eliminating or nullifying the repulsive energy barrier that exists between two particles. The second step-particle transport (flocculation)-is concerned with inducing interparticle contacts by Brownian motion of t h e colloidal particles (perikinetic flocculation), the effects of velocity gradients within the suspending fluid (orthokinetic flocculation). or differential settling velocities of suspended particles. Particle destabilization can be accomplished by four distinct mechanisms: adsorption to produce charge neutralization, adsorption to permit interparticle bridging, enmeshment in a precipitate of a metal hydroxide, and double layer compression. Colloidal suspensions are quite stable in freshwaters owing to the repulsion t h a t exists between the electrical double layers surrounding the negatively charged particles-solvation effects and adsorbed organics may also contribute to colloid stability. In conventional water and waste water treatment practice, destabilization is accomplished by utilizing the first three mechanisms listed above. In an estuary, the conditions are conducive for destabilization to be accomplished by double-layer compression. Experimental Methods. Three different clay minerals -kaolinite, illite, and montmorillonite-supplied by
Here n is the concentration of particles a t time t (parti-
Table I. Composition of Buffered NaCl Solutions Species
Ion concn, mol/l.
Ion concn,
I o n concn, rnol/l.
NaHCO3CI-
0.300 0.002 0.298
0.090 0.002 0.088
Ionic strength
0.300
0.090
0.050 0.002 0.048 0.050
rnol/l.
Table II. Composition of Synthetic Estuarine Solutions Species
NaMg2+ Ca2+ K+ Sr*+ H30CI-
so42HCOBB r-
FOHI o n i c strengths Salinity, ppt
Ion concn, mol/l.
Ion concn, rnol/l.
Ion concn,
0.231 0.026 0.005 0.005 7.5 X 10W 10-8 0.268 0.014 0.002 5 x 10-4 3.6 x 10-5
0.0577 0.0065 1.25 x lo-: 1.25 x 101.87 x lo-. 10-8 0.067 0.0035 0.002 1.25 x 10+ 9 x 10-6
0.0231 0.0026 5 x 10-4 5 x 10-4 7.5 x 10-6 10-8 0.0268 0.0014 0.002 5 x 10-3 3.6 x
mol/l.
10-6
10-6
10-6
0.343 17.5
0.087 4.4
0.036 1.8
a Calculations of ionic s t r e n g t h were n o t a d j u s t e d for i o n p a i r i n g
Volume8, Number 1, January 1974
59
4x
108,
I
1
1
4
0 = 52.4 s e c - I
-
h ' ,
G
=
(;)
I/?
(3)
0.05M
= . . i
\ v)
-
\'A,
E
a -
torque on the paddle shaft. Camp and Stein (1943) related the total energy input into a fluid to the root-meansquare velocity gradient, G :
I
= 8.81 x 10-6
\-
2x10~
0.09 M
.-u
where t is the total energy dissipated per unit time and unit fluid mass and v is t h e kinematic viscosity. The rootmean-square velocity gradient can be evaluated by expressing Equation 3 in the following form:
($)
1/2
4L
G
0
a c
0
20
IO
30
40
TIME, m i n Figure 1. Coagulation kinetics of kaolinite in buffered NaCl s o b
tions at three ionic strengths
SXlO'
7
k
=
(4)
where I" is the net torque (dyne cm), o is the angular velocity of the rotating paddle (radians per sec), is the fluid viscosity (gicm sec), and .V is the fluid volume (cm3). Experimental Results. A series of experiments was conducted to determine the rates of coagulation of different clays in various solutions. Figure 1 illustrates the effect of increasing ionic strength on the coagulation kinetics of kaolinite in buffered NaCl solutions while Figure 2 illustrates the effect of increasing ionic strength on the coagulation kinetics of kaolinite in synthetic estuarine solutions. These figures show that the rate of coagulation increases with increasing ionic strength. Similar results were obtained for illite and montmorillonite clay suspensions. Stability values. CY, were evaluated from the slopes of the least-squares regression lines and are summarized in Tables I11 and 11'. Pamlico Riiier Estuary Studies
4
-
8.67 x IO-' 0 - 52.3 sac-1
4 I IO'
0
IO
20
30
40
TIME, m i n Figure 2. Coagulation kinetics of kaolinite in synthetic estuarine
solutions at three ionic strengths
cles,cm3), cy is the stability factor, 4 is the volume of colloidal particles per unit volume of suspension, and G is t h e root-mean-square velocity gradient (sec-I). A completely destabilized suspension has a stability value of one (cy = 1); stable suspensions are characterized by a < < 1. Integration of Equation 1 yields the following:
The Pamlico River Estuary flows in a n approximate northwest-southeast direction from Washington, N.C., to the vicinity of Pamlico Point where it enters Pamlico Sound (Figure 3). The Pamlico River Estuary is a shallow well-mixed estuary with a maximum width of about 8 miles (12.9 k m ) and an average depth of about 10.5 ft (3.2 m ) . The outer banks of Korth Carolina have a damping effect on the lunar tides; consequently, the lunar tide is only about 6 in. (15.3 cm) and is greatly overshadowed by wind tides of u p to 3 ft. (0.915 m ) . The salinity varies from less than 0.5ppt (parts per thousand) near Washington to around 16 ppt at the mouth as shown in Figure 4. T h e salinity values are based on bimonthly measurements made from J u n e 1968 to December 1970 by Hobbie (Hobbie. 1970; Hobbie et al., 1972) and from Water Resources Data for Korth Carolina (1965-69). The water is well mixed owing to the shallowness of the estuary yielding ir-
Table 111. Stability Values. Buffered NaCl Solutions Stability value, Ionic strength
where no and n are the particle concentrations a t time equal to 0 (zero) and a t time t In this investigation stability values, cy, were evaluated from kinetic d a t a in which the change in clay particle number over time was observed by using microscopic counting and Coulter Counter techniques. For suspensions which are initially monodispersed, this model is quite adequate to describe the early stages of coagulation. T h e rate studies were conducted in a 2-liter reactor. A torquemeter (Power Instrument Co., Skokie, Ill.) was coupled between a dc stirring motor and t h e stirring shaft so that the stirring power input to the reactor could be determined by direct measurement of the 60
Environmental Science & Technology
0.05~ 0.09M 0.3M
01"
Montmorillonite
'Kaolinite
Illite
0.075 i 0.009 0.089& 0.006 0.125+ 0.005
0.0245 =t0.007 0.0308 I0.004 0 . 0 7 2 4 ~0.007
0.0128 =t0.003 0.0275 i 0.005 0.0455 0.005
+
U T h e 95% confidence interval for a is indicated.
Table IV. Stability Values. Synthetic Estuarine Solutions Stability value, a" Ionic strength
Montmorillonite
0.036M 0.087M 0.343M
0.0943 i 0.003 0.113 0.019 0.148 0.006
+ +
Kaolinite
0.0445 0.0915 0.138
+ 0.002 0.006 + 0.009 Z!C
T h e 95% confidence interval for a is indicated.
Illite
0.0180 = 0.003 0.0701 i 0.007 0.0740 i 0.009
0
5
IO
20
30
Stations
- / Sampling
16
I
3
MILES
12
5 * 4 -
-
RR
-4
12
4
NAUTICAL
Figure 3. Map of the Pamlico River Estuary showing approximate sediment sampling stations
20
MILES
36
28
DOWNSTREAM
Figure 4. Salinity of t h e surface waters of the Pamlico River Estuary as a function of the distance downstream from t h e railroad bridge at Washington, N.C. (Hobbie, 1970; Hobbie et al., 1972; and Water Resources Data for North Carolina, 1965-69) T h e position of sediment-sampling stations ('3, -12. and ~ 1 8 is) indicated
regular stratification with no permanent salt wedge (Hobhie. 1970). An E k m a n dredge was used to collect bottom sediments from along the center of the Pamlico River Estuary (Figure 3). Formaldehyde was used during collection to retard biological activity and t h e sediment samples were frozen in the field with Dry Ice. In the laboratory. several representative sediment samples were selected and subjected to X-ray diffraction and coagulation kinetic experiments. Clay Mineral Composition of Pamlico Sediments. T h e clay size fraction ( < 2 ,urn) of t h e sediment samples was isolated by centrifugation and t h e clay mineral composition determined by X-ray diffraction. The relative amounts of t h e clay minerals in each sample were calculated by comparing the ratios of weighted basal peak areas (Pierce and Siegel. 1969; Freas, 1962). The results of such techniques are "semiquantitative" (k107c of the total clay fraction) at best; however. the results can be used to indicate trends in the clay mineral composition of the sediments. The composition of the clay fraction as a function of sediment sample location is plotted in Figure 5. Coagulation Kinetics of Pamlico Sediments. Three Pamlico sediments (s3. -12. and ~18) were selected and used in coagulation kinetic experiments. The sediment sampling stations for these sediments are indicated in Figures 3 and 4. Pamlico sediment s3 was obtained from the river above Washington. T . C . . where the salinity is less t h a n 0.5 ppt. Sediments 212 a n d 118 were obtained from the estuary. Pamlico sediments r3, 212. and -18 will be referred to as t h e freshwater, upper estuary, and lower estuary sediments. respectively. T h e methods employed to evaluate stability values from coagulation kinetics experiments have been described previously. Stability values were obtained &oreach sediment a t three different ionic strengths using synthetic estuarine solutions (Table 11). The results are shown in Figure 6 and indicate that the upstream sediments are relatively unstable (higher N values) compared to downstream sediments.
I
z
I
I
I
I
Chlorite
I-
0
a -
I
a
0.12
IJT
Discussion L'erwey-Overbeek and Derjaguin-Landau (1948) have independently developed a quantitative theory-VODL theory-in which t h e stability of lyophobic colloids is treated in terms of the energy changes which take place when colloidal particles approach one another. According to the L'ODL theory, t h e degree of compression of the double-layer thickness is governed by the concentration
-
0 0
4
8
I2
16
SALINITY,
ppt Figure 6. Stability value ( a )as a function of salinity for clay size fraction of Pamlico sediments F r e s h water sediment, -3; upper estuary sediment, -12; a n d lower estuary s e d i m e n t , =18
Volume8, Number 1, January 1974
61
Table V. Comparison of
CY
Values
Investigators
H a h n and Stumm H a h n a n d Stumm Swift and Friedlander Swift and Friedlander Birkner a n d Morgan This research This research ‘I
Coagulant
AI(III) AI(I I I )
NaCI, 1M
NaCI, 1M NaCI, 1M NaCl solutions (0.05-0.3M) Estuarine solutions (0,036-0.343M)
Colloid
Type o f flocculation
LI
Silica Silica Polystyrene latex Polystyrene latex Polystyrene latex Clay minerals
Perikinetic Orthokinetic Perikinetic
0.01 to 0 , l a 0.01 to 0.P
Orthokinetic
0.364
Ortho ki net ic
0.344, 0.448
Orthokinetic
0.012-0.12*
Clay minerals
Orthokinetic
0.02-0.15’
0.375
Values in t h e order of 0.01-0.1 d e p e n d i n g u p o n c o a g u l a n t dosage a n d s o l u t i o n p H .
b Values of t h i s order d e p e n d i n g u p o n ionic s t r e n g t h .
and valence of the counterions. The degree of colloidal destabilization is improved as the ionic strength and the charge o f t h e counterions increase. The results of the laboratory phase of the coagulation kinetic experiments have been summarized in Tables I11 and IV. The stability values, cy. for t h e clays indicate improved particle destabilization in solutions of increasing ionic strength-completely destabilized clays would be characterized by cy = 1. A comparison of cy values obtained for clays destabilized with NaCl solutions vs. estuarine solutions indicates lower cy values for clays destabilized with KaCl. This is to be expected since the estuarine solutions contain divalent cations which are more effective than monovalent cations in accomplishing particle destabilization. These results-effects of ionic strength and valency of cations-are in qualitative accord with the theory of particle destabilization by double layer compression (VODL theory). It can be concluded from the laboratory phase of this study that the stability factor, cy: depends upon the type of clay mineral, the ionic strength of the destabilizing solution, and the composition of the solution (valency of counterions). This is in agreement with the results of Hahn and S t u m m (1968) who concluded t h a t colloidal stability is dependent upon chemical solution parameters. Table V compares stability values obtained from this research with values obtained by other investigators (Hahn and S t u m m , 1968; Swift and Friedlander, 1964; Birkner and Morgan, 1968). The stability values for the clays vary over a range typical of values reported for other colloids. The results of the kinetic studies (Tables I11 and I\’) indicate t h a t the stability of the clays is as follows: Illite is more stable (lower cy values) than kaolinite which is more stable than montmorillonite. The clay mineral composition of the Pamlico sediments as a function of sediment sample location is plotted in Figure 5 . Kaolinite is the dominant clay in the upper end of the estuary where salinity is lowest and decreases toward the mouth where salinity is highest. Illite occurs in minor amounts in the upper end and increases toward the mouth, while montmorillonite is present in minor amounts along the entire length of the estuary. Chlorite and chlorite-like integrade clay (14 A) comprise the remainder of the clay fraction. T h e distribution of kaolinite and illite in the Pamlico sediments can be explained by coagulation. In the laboratory kinetic studies reported earlier, kaolinite was observed to be unstable relative to illite. Consequently. under similar physical conditions, kaolinite would be expected to aggregate more rapidly than illite and be deposited upstream from illite. An independent test was made of the hypothesis that the distri62
Environmental Science & Technology
bution of kaolinite and illite in the Pamlico sediments can be explained by the relative stability of the clays. The results are presented in Figure 6 in which coagulation rate studies were used to determine the stability values for three Pamlico sediments. For any given salinity (or ionic strength) cy is highest for the freshwater sediment-=3-and lowest for the lower estuary sediment~ 1 8It. can be concluded that the stability of the Pamlico sediments is as follows: =18 is more stable (lower cy values) than =12 which is more stable than -3. The sediments deposited upstream have undergone particle aggregation more rapidly than those deposited downstream, which is in agreement with the clay mineral distribution of kaolinite and illite for the Pamlico sediments. A model is developed in this paper in which the transport of colloidal suspensions from freshwaters into estuaries is accompanied by a reduction in particle stability resulting in the coagulation and deposition of these materials in estuaries. Hahn and Stumm (1970) have presented models for coagulation in natural waters to describe changes in the suspended phase. However. Hahn and S t u m m used theoretical estimates of stability for clays which are transported into estuaries. This contrasts with our work in which stability values for the clay minerals were determined experimentally. The coagulation and deposition of suspended matter can have significant effects on water quality. Cohesive sediments such as clays and silts are responsible for the shoaling of estuarine channels, the formation of deltas, and the persistence of turbidity currents in estuaries. A substantial portion of the BOD in the effluent from conventional secondary waste treatment plants is colloidal (Bishop et al., 1965). These organic colloids. like the inorganic clay minerals, may also coagulate in estuaries. Gross et al. (1971) reported that approximately 40% of the bottom of New York Harbor is covered by fine-grained wastes with sewage solids being a major constituent. These deposits can have several adverse effects on water quality. including oxygen demand, release of bacteria and viruses. and shoaling of the waterway. Literature Cited
Birkner. F. B.. Morean. J. ,J.. J Amer. W a t e r W o r k s Ass.. 60. ” 175-91 (1968). Bishop. D. F.. Marshall. L. S., O’Farrell. T. P.. Dean. R. B.. 0 ’ Connor, B.. Dobbs, R. A , . Griggs. S. H.. Villers. R. V.. J . Water Poilut. Contr. Fed.. 39, 188-203 11967). Camp. T. R.. Stein, P. C.. J . Boston .5oc. Cii). Eng.. XXX. 21937. (1943). Edzaald. .J. K . “Coaeulation in Estuaries.” University of Xorth Carolina Sea Grait Program. Publication UNC-SG-72-06, Chapel Hill. N.C.. 1972.
Etter, R. .J., Hoyer. R. P., Partheniades. E.. Kennedy. .J. F.. J . H l d r a u ! . Dit',, A m e r . .\oc. C i i . E n g . , 91 (HT6). 1139-52 (1968). Freas, D . H., Geol. .5oc. A m e r . B u l l . , 73, 1341-63 (1962). Gross. iL1. G., Black, J. A , , Kalin. R. J . . Schramel. J. R.. Smith. R. N.,"Survey of Marine LVaste Deposits. Sew Tork hletropolitan Region." Tech. Rept. S o . 8. Marine Sciences Research Center, State University of New York. Stony Brook, N.Y.. 1971. Hahn, H. H., Stumm, LV., A m e r . J. .\ci.. 268, 354-68 (1970). Hahn, H. H., Stumm. \V,. J . Colloid lnter,face h'ci., 28, 134-44 (1968). Hobbie, J. E.. "Hydrography of the Pamlico River Estuary, N.C.," Rept. !io. 39 of the LVater Resources Research Institute of the University of North Carolina, Raleigh. N.C.. 1970. Hobbie, J . E.. Copeland. B. J . . Harrison, \V, C.. "Sutrients in the Pamlico River Estuary, N.C.. 1969-71." R e p . No. 76 of the LVater Resources Research Institute of the Cniversity of Sorth Carolina, Raleigh, K.C.. 1972. Ippen. A . T.. Ed.. "Estuary and Coastline Hydrodynamics." McGraw-Hill. Kew York. N.Y.. 1966. Krone. R. B.. "Flume Studies of the Transport of Sediment in Estuarial Shoaling Processes," Final Report to San Francisco District. U.S. Army Corps of Engineers. University of California, Berkeley. Calif.. 1962.
Lyman. .J.. Fleming. R. H.. J . .Warine Kea., 3 , 134 (19401. O'Melia. C . R.. "Coagulation and Flocculation." "Physicochemical Processes for LVater Quality Control." \Valter .I. \Veber. .Jr.. \Viley-Interscience. S e w York. S . Y . , 1972, Partheniades. E,. J . H j d r a u i . DIL'..A m e r , \ o c . C ' i i E n z . , 91. (HTli. 105-39 (1965). Pierce. .J. \V.. Siegel. F. R.. J . .\ediment. Petroiog:. 39, 187-93 i 1969). Smoluchowski. >I., Z. P h i s . C'hem.. 92, 129-68 (I917l. Swift. D. L.. Friedlander. S. K.. J . Colloid .\ci.. 19, 621-17(1961). Verwey. E. J . LV., Overbeek. J. Th. G.. "Theory of the Stability of Lyophobic Colloids." Elsevier. New York. S . Y . . 1948. "LVater Resources Data for Sorth Carolina. Part 2, LVater Quality Records." U.S. Department of the Interior. Geological Survey. j 196j-69). Receii,ed for r e i i e u ,March 29. Z W I . 'Accepted .\eptember 24, l.Yi3. Presented at t h e 45th A n n u a l Conference o f t h e W a t e r l'oilution Control Federation, A t l a n t a . G a . , October I O . l:)i2. 7'hts research was supported i n part b\ t h e Office ~f .\ea Grant. .Yotional .\cience Foundation u n d e r Grant AYo. GH-103 and b\, a L'.S. Public H e a l t h SerL'ice Traineeship. Grant ,Vo. GP-LA03.4HO0509-06.
Recovery of Hydrogen Fluoride Fumes on Alumina in Aluminum Smelting C. Norman Cochran Alcoa Laboratories, Alcoa Technical Center, Alcoa Center, Pa. 15069
Calcined aluminas with widely ranging surface areas rapidly chemisorb a monolayer of H F at partial pressures of a few microns in A1 smelting fumes at 120°C. The chemisorbed layer contains two H F molecules per surface A1203 molecule and converts to crystalline AlF3 above 300°C. Additional H F physisorbed at higher partial pressures is mostly desorbed upon heating above 120°C. Adsorption isotherms were derived from data for processes in which A1 smelting fumes are contacted with calcined A1203 for recovery of fluorides. T h e fluoride-containing A1203 is fed t o the A1 smelting cell for electrolysis to close t h e fluoride fume generation and recovery loop. This has lowered the A1F3 requirements of A1 smelting to essentially that for converting the Na2O impurity (introduced with the A1203) to fluoride bath of the desired composition. T h e H F is generated near the 9 7 5 T smelting temperature by reaction of AIF3-containing species with moisture. The reverse reaction favored a t lower temperatures is the basis for the recovery process. Chemisorption on calcined aluminas is receiving much attention as t h e capture mechanism in a closed loop system for recycling hydrogen fluoride from A1 smelting fumes (Cochran et al., 1970; Colpitts. 1972: Cook et al.. 1971; Lobos et al.. 1971; Minchin et al.. 1972: Muhlrad and Chauvineau, 1973; Nasmith and Miller. 1973; Nielsen and Kielback. 19'72; K'esenberg, 1972). This recovery syst e m is more direct than t h e previously favored wet scrubbing operations. It avoids neutralization, precipitation. chemical conversion. and drying usually required before the fluoride can he recycled t o smelting cells. Some of the basic chemistry of this recovery operation and its relationship to fume generation in the smelting cell will he presented.
Recoi,er? and
H?drol? sis Equiiibria
The ultimate overall reaction in recovering H F with
A1203 is to form AlF3 and H 2 0 by t h e favorable equilibriu m constant shown below for 127°C.
log K9:;y
1.64)
The original source of the H F in smelting is essentially the reverse reaction in which AlF3-containing bath species react with H 2 0 vapor in accord with the favorable rightto-left reaction equilibrium constant for 977°C above. Of the bath components. sodium fluoride is less susceptible to hydrolysis than AlF3. Hydrolysis of pure AlF3 is important because AlF3 is often added to pots to make up fluoride losses. As might be expected, KasAlFC is intermediate between NaF and AlF3 in its tendency to hydrolyze. Generation of H F becomes more severe as temperature and H 2 0 partial pressure increase and alumina content decreases.
Sources of H 2 0 t o Form
HF
Water can be introduced with the A1203 either as a d sorbed H 2 0 or as constitutional H2O. The factors governing the amount of adsorbed H 2 0 are relative humidity. surface area, and degree of exposure of the A1203. The constitutional H 2 0 is determined by calcination conditions at the Baser plants. In pilot cells, only about 5% of the total H 2 0 of A1203 reacts with bath to form H F (Henry, 1963). Probably the most important sources of H 2 0 for hydrolysis are atmospheric moisture in the pot rooms a n d H20 from burning of hydrocarbons or H2 evolved from the anodes. Absolute humidity rather than relative humidity would govern the equilibrium of the reaction with H20 vapor.
Plant Procesaeb Most of the sorption studies were conducted in operating plant installations of the processes represented in Figures 1 and 2. Volume 8, Number 1 , January 1974
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