Cationic Concentration by Freezing - Advances in Chemistry (ACS

8 Cationic Concentration by Freezing B. A . M A L O and R. A . B A K E R

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Carnegie Mellon University, Mellon Institute, Pittsburgh, Pa.

The results of an investigation of cationic separation during the freezing process as it applies to concentration of trace contaminants from water are reported. The cations includ ing Fe, Cu, Zn, Mn, Pb, Ni, Ca, Mg, and Κ were studied at concentrations of 0.1 to 10 mg./liter. For any given cation, recovery does not vary over the experimental concentration range. However, selective ion incorporation into the ice matrix was observed when demineralized, laboratory pre pared solutions and highly mineralized, naturally buffered waters were concentrated. Increasing mixing rate up to some limiting value increases cationic recovery. Whether initial pH increases or decreases, recovery depends on the nature of the cation. Alkali metals (K, Ca, and Mg) concentrate best at acid pH. Heavy metal cations (Pb, Ni, and Cu) concen trate best at alkaline pH. Generally, increasing initial total dissolved solids decreases cationic recovery.

In order to separate, identify, and quantify trace contaminants in water, the analyst must employ concentration procedures preliminary to analyses. Continuing studies i n these laboratories have examined the freeze concentration techniques for μg/liter to mg./liter levels of organics from aqueous solution. It was observed that inorganic salts significantly affect organic recovery efficiency. A n investigation to determine the nature of this effect afforded an opportunity simultaneously to examine i n organic cationic separation during the freezing process. The subject of salt effects on the freezing process is not new. Nevertheless, despite many years of study, it is still an inadequately understood process (12). Recent applications of the freeze concentration technique to inorganic recovery from water have been reported (2, 9, 10, 16). Many of these studies were prompted b y a communication (15) 149

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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TRACE INORGANICS I N W A T E R

which renewed interest in freezing of water as a means of concentration, particularly for recovery of volatile, heat-labile substances. The objective of this paper is to describe preliminary results obtained from the investigation of the process of cationic separation during the freezing process as it applies to concentration of trace contaminants from water. VOLUME 20

10

5

40 VOLUME

RATIO

4

3

60 REMAINING,

80 ML

Figure 1. Influence of selective ion incorporation on the freeze concentration efficiency of metal cations Matrix: U.S. Mean Water. Mixing Rate = 80 r.p.m. Bath Temperature — —12°C. Initial: volume = 200 ml; pH — 6.6; specific conductance = 225 ^mhos/cm.; cation concentration, mg. I liter = 7.5 Ca, 96. Cu, 0.2 Fe and 9.7 Pb Experimental The experimental procedure for freeze concentration used i n this study has been detailed elsewhere (3). In brief, a specific sample volume is chilled to approximately 1°C. and transferred to a round-bottom flask that has been prechilled and seeded with a few ice crystals. The flask capacity is selected to exceed two sample volumes. The flask is immedi ately attached to a mixing device and lowered into an ice-salt bath main tained at approximately —12°C. The flask is rotated at a predetermined rate until the desired volume, judged by experience and estimated by elapsed time, has been frozen. The cations are concentrated in the


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Cationic Concentration

residual liquid. The flask is disconnected and the residual liquid is trans ferred and its volume measured. The ice is melted and both liquid and ice (when possible) are analyzed for p H , specific conductance ana cations by atomic absorption spectrometry. The efficiency of recovery in percent is calculated from: _ „ chemical concentration ratio _ % Recovery = = X 100 volume ratio w

Λ Λ

:

= [ « W C

i

(

1

)

) / ( V t / V , ) ] 100

Cz,(i) = initial solute concentration in liquid, mg./l. CL(2) = final solute concentration in liquid, mg./l. V

x

= initial sample volume, ml.

V

2

= final volume of liquid, ml.

The experimental aqueous matrices included deionized-distilled water, laboratory-prepared phosphate buffer solutions, U.S. mean water (1), and tap waters. U.S. mean water is a solution of specified mineral content which may be prepared and used as a reference standard. These aqueous systems were supplemented with known quantities of F e , C u , Zn, M n , Pb, N i , C a , M g , and Κ singly and in mixtures. Cations with the exception of F e and Κ were added as nitrates. F e and Κ were added as chlorides. The freezing rate is controlled b y the vessel rotation rate and bath temperature. Specific conductance is used as an indicator of the dissolved solids concentration, although it is recognized that this property is influenced b y the hydrogen ion concentration as well as the nature of the dissolved constituents. Results The effect of certain factors on the recovery efficiency of various cations from water b y freezing was studied. These included the mixing rate, initial p H , specific conductance or total dissolved solids, cation con centration and initial and final volume. When a mixture of cations in water is concentrated by freezing, indi vidual recovery efficiency usually varies. This is illustrated by the result of a single-stage freeze concentration shown i n Figure 1. In this case, a quaternary mixture of cations was recovered i n the sequence C a > C u > P b > F e . These cations were initially dissolved i n a U.S. mean water. Initial conditions were: p H , 6.6; specific conductance, 225 /xmhos/cm.; and the cations at 7.5 to 9.7 mg./liter. The variation in cationic recovery in the concentrated liquid is caused by selective ion incorporation i n the residual ice. Because of this selective incorporation, it is not possible to predict the final abundance of a cation relative to the other components in the liquid phase. The recovery efficiency of the four cations cited is


152


unique to this test. It applies only for this water and the operating con ditions specified.

VOLUME

Figure 2.

RATIO

The effect of mixing rate on the recovery of Mn from a deionized-distilled water matrix

Initial: volume = 200 ml; pH = 3.8; specific conductance = 164 μmhos/cm.; Mn = 1.0 mg./liter W i t h the procedure practiced i n these studies, mixing was varied by changing the rotation rate of the flask containing the sample. N o internal stirrers were used. The effect of mixing, or rotation, on cationic recovery efficiency from a deionized-distilled water and a tap water is presented i n Figures 2 and 3 respectively. These waters were initially supplemented by m g . / l . concentrations of M n , Fe, Pb, N i , C u , and Zn. Only the manga nese recovery data are presented since the effect was similar for all cations. In these illustrations, the solid, diagonal line represents ideal or total concentration efficiency—i.e., the volume and chemical concentra tion ratios are identical. The amount of departure of the experimental data from this line is a measure of loss of efficiency. W i t h both waters, mixing improved cationic recovery over quiescent freezing. However, increasing rotation rate from 80 to 260 r.p.m. d i d not further improve


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Cationic

Concentration

153

separation. This effect was previously observed i n the freeze concentra tion of organics (4). These results also demonstrate the interrelationship of initial total dissolved solids content, mixing rate and recovery efficiency. For example, manganese recovery from the 660 ^mhos/cm. tap water at 260 r.p.m. is comparable i n efficiency to that at 0 r.p.m. from the 164 ftmhos/cm. deionized-distilled water. The elapsed time required to freeze a given aqueous volume at —12°C. bath temperature is twice as great at 80 r.p.m. than at 260 r.p.m. ( 3 ) . Since increased mixing tends to improve recovery, field applications of this procedure should be made at the high est practical mixing rate. T h e influence of dissolved solids content is treated in subsequent paragraphs.

VOLUME

RATIO

Figure 3.

The effect of mixing rate on the recovery of Mn from tap water Initial: volume = 200 ml; pH = 6.0; specific conductance = 660 μmhos/cm.; Mn = 1.0 mg./liter The p H of the ice-liquid system influences cationic separation and recovery. Figure 4 depicts the effect of p H on recovery of manganese from a tap water which was supplemented by 1 mg./liter quantities of M n , Fe, C u , Zn, P b , and N i . The initial p H of 6.1 was adjusted to 3.6 and 10.2 with H C 1 and N a O H respectively. The manganese recovery was slightly greater at alkaline p H for comparable volume ratios. The p H effect is also shown in Figure 5 for lead, nickel, and copper and in Figure


154

T R A C E INORGANICS I N W A T E R

6 for calcium, magnesium and potassium. Improved recovery of P b , N i , and C u i n the liquid phase is favored b y increased alkalinity while i n creased acidity enhances C a , M g , and Κ separation. These observations are particularly applicable to volume ratios greater than 5:1. Little dif ference is evident at lower volume ratios. The consistency of selective re covery order for these ternary cationic mixtures over a range of volume ratios confirms the phenomenon of selective incorporation.

VOLUME

RATIO

Figure 4.

The effect of pH on the recovery of Mn from a tap water Mixing Rate = 80 r.p.m. Initial: volume — 200 ml; Mn = 1.0 mg./liter; specific conductance = 955, 655, and 919 μητηοε/αη. for pH = 3.6, 6.1 and 10.2 respectively Organic recovery efficiency from aqueous solution b y freeze concen tration has been reported (4) to decrease as the specific conductance or total dissolved solids increased. A similar effect has been observed for the recovery of heavy metals from a variety of aqueous solutions. Figure 7 illustrates the recovery-suppressing effect of dissolved solids on copper as measured by specific conductance i n the range of 40 to 40,000 ftmhos/cm. The matrices include deionized-distilled water, 10" to 10~ M phosphate buffers and tap waters originally at p H 4 ± 0.5, and 0.12N (approximate) H C 1 solution at p H 1.4. The recovery of other heavy metal cations follows i n a similar manner. 3


4

8.

M A L O A N D BAKER

40

155


0 20 40 0 VOLUME REMAINING, ML

20

40

60

Figure 5. The effect of pH on the recovery of Pb, Ni, and Cu from tap water at low residual volumes Mixing Rate — 80 r.p.m. Initial: volume — 200 ml.; cation concentration, mg./liter = 2.3 Pb, 1.0 Ni, and 1.0 Cu; specific conductance = 955, 680, and 919 μητηοε/αη. for pH = 3, 7 and 10 respectively

100

20

40

0 20 40 0 VOLUME REMAINING, ML

20

40

Figure 6. The effect of pH on the recovery of Ca, Mg, and Κ from tap water at low residual volumes Mixing Rate = 80 r.p.m. Initial: volume = 200 ml; cation concen tration, mg./liter = 28.0 Ca, 6.3 Mg and 1.8 K; specific conductance = 1080, 360 and 413 ^mhos/cm. for pH ^ 3, 7, and 10 respectively


156


Figure 7. The effect of specific conductance on the recovery of Cu Mixing Rate = 80 r.p.m. Initial: volume = 200 ml; pH ^ 4; Cu = 1.0 mg./liter Single-stage freeze concentration recovery efficiency was not affected by initial cationic content i n the range of 0.1 to 10 mg./liter. Figure 8 shows this for copper recovery from a synthetic aqueous solution prepared by adding equal quantities of copper and lead to U.S. mean water. This water contained C a , M g , Fe, M n , N a , and K . The recovery of other heavy and alkali metal cations followed that of copper. The initial volume employed for a single-stage freeze concentration is not in itself a controlling factor i n contaminant recovery efficiency. It does limit the extent of volume concentration which may be achieved at maximum efficiency. There is a critical final volume of unfrozen liquid below which recovery is impaired. This limiting range varies for the water and operating conditions being used. This is a mechanical-experimental limitation for the procedure being used and involves the residual liquid volume held on the ice, flask, etc. The ice was not washed i n these tests. Some concentrated liquid is expected to freeze on the ice surface during the sample transfer operation. A similar observation was reported b y Baker (3) who suggested a limiting final volume of 30 ml. per unwashed


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157

single-stage freeze concentration from waters of low dissolved solids con tent for maximum recovery of trace organics. A representative cationic recovery-volume ratio relationship is shown i n Figure 9 for lead. Other heavy metals give similar results. Here efficiency loss begins to occur between 50 and 70 m l . residual volume. The initial volume selected per single-stage freeze is determined by considering the desired volume ratio and the hmiting final volume. For high volume ratios larger initial sample volumes are required. The data point indicated by the triangle ( Δ ) at 30 ml. residual volume resulted from a single stage freeze of 1000 m l . This represents a volume ratio of 33.3 and it was performed with the same efficiency, 7 5 % , as a 200 ml. freeze at a volume ratio of 8.7. The time required for the 1000 m l . freeze was 55 minutes, while the 200 m l . freeze took 21 minutes. Both were performed at 80 r.p.m. mixing rate and —12°C. bath temperature. A three-liter, round-bottom flask was used for the 1000 m l . initial volume tests. A n alternative to the use of greater initial volumes and associated increased mixing load is the cascade freeze concentration technique ( 3 ) . This procedure requires more time than the high volume single-stage freeze, but it permits almost any desired level of concentration at maximum efficiency. The data point represented b y the inverted triangle ( V ) at 74 m l . residual volume is a 27.0:1 volume

5 10 VOLUME RATIO

Figure 8.

The effect of initial Cu concentration on its recovery from U. S. mean water

Mixing Rate = 80 r.p.m. Initial: volume = 200 ml; pH — 6.6 to 7.1; specific conductance = 200 to 226 μmhos/cm.


158

T R A C E INORGANICS I N W A T E R 100

Lu

» 80 UJ Q.

> 60 h

UJ > 40 Q: UJ

ο • Δ V

s

S

20 h

û.

20

INITIAL VOLUME 2 0 0 ML 4 0 0 ML 1000 ML 2000 ML (CASCADE)

100

40 60 80 VOLUME REMAINING, ML

Figure 9. The effect of initial sample volume on the recovery of Pb from U. S. mean water Mixing Rate = 80 r.p.m. Initial: pH = 6.6 to 7.35; specific conductance = 190 to 226 iimhos/cm.; Pb = 1.8 to 9.7 mg./liter 400

400

400

400

400

ML

ML

ML

ML

ML

1

1

106 M L

1

135 M L

1

STEP I

STEP JT

1

1

102 M L

81 M L

521 ML

74 ML

1

1 97 M L

1

V O L U M E R A T I O Ï =3.84

VOLUME RATI0 =7.04 n

OVERALL VOLUME RATIO =27.0

Figure 10. An experimental, two-step cascade scheme for cation recovery from a 2000 ml. initial volume concentration yielding maximum recovery for P b (92% ). This was an 80 r.p.m., two-stage cascade freeze involving concentration of five 400 m l .


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individual volumes to approximately 100 ml. (4:1), compositing the con centrates and freezing this composite to 74 ml. residual volume (Figure 10). The time required for the cascade freeze was 220 minutes. Availa bility of multiple mixing devices for simultaneous concentration of a number of single-stages and higher mixing capacity would markedly shorten this time. A multi-step cascade freeze concentration such as that i n Figure 10 maximizes recovery efficiency and yields sufficient con centrate volume for extensive inorganic and organic analyses. 40

I

•

•

ι

ι

F

Τ

Τ

τ

-ρ

ill

30 h

RESIDUAL VOLUME y*!%VC ^ ^ ^ R E S I D U A L VOLUME >70 ML

10 h

1

1

1 1 1 1000 INITIAL VOLUME, ML 1

1

1

•

2000

Figure 11. Limiting volume concentration ratio for maximum recovery efficiency as a function of initial sample volume A graphical representation, Figure 11, shows the limiting volume concentration ratio for maximum recovery efficiency as a function of initial sample volume. F o r any given sample volume, recovery is maxi mum over the volume ratio range i n the crosshatched area. Further i n crease i n volume ratio yielding residual volumes lower than 50 m l . w i l l appreciably decrease recovery. This limiting volume concept applies to any given single-stage freeze concentration whether it is ( a ) separate or ( b ) part of a multi-step cascade sequence. Discussion Workman and Reynolds (18) originally documented observations of charge separation and associated electrical potential at the growing ice surface. This phenomenon is now called the Workman-Reynolds Effect.


160


Subsequent investigations by others (7, 8, 11) contributed further to understanding of this effect although much remains to be explored. Among the factors examined were ionic radius, electronegativity, nature and concentration of solutes and freezing rate. These experiments were idealized and were conducted with selected systems in apparatus permit ting measurements such as interfacial potential, current, and freezing rate when ice growth was unidirectional. The preliminary results reported here were obtained during a study of macrovariables with apparatus i n which controls were not as rigorous. Thus, some of the postulates ad vanced in other studies are not readily applicable. Selective cationic recovery is suggested from the results (Figures 1, 5, and 6) but the order of recovery was not predictable over the range from 0.1 to 10 mg./liter initial concentration. Even if the physical parameters of freezing rate, temperature gradient across the ice-liquid interface and orientation of crystallographic axis of the ice had been controlled, a number of other factors would have to be considered if the results are to be adequately explained. These include: proton conductance; dipole orientation of water molecules at the interface; ionic entrapment in the ice lattice; and ionic diffusion. The processes may function singly or in combination, depend ing upon the constituents of the solution and their concentration. Parreira and E y d t (13), for example, speculated that in the range of 10" to 10" M N a C l , proton conduction may originate the freezing current whereas dipole orientation is responsible for the buildup of the large freezing potential observed. For this case, ionic entrapment and dif fusion processes may have been operating, but their contribution was estimated to be slight. 7

5

Nucleation, the first step in crystallization (12), is induced by the practice of flask seeding. The second step, ice formation or growth is of greatest importance in the concentration procedure. The manner in which latent heat of fusion is conducted away from the ice-liquid interface determines the nature of the resulting ice surface. Thermal conduction away from the interface through the ice produces ice which is smooth on an atomic scale with propagation at the edge of the crystalline basal plane proceeding laterally. Lateral growth rate is greater than that per pendicular to the basal plane. Thermal conduction may also be into the liquid phase if this phase is supercooled. A very branched, uneven growth then occurs. This dendritic or branched morphology (5) is strongly i n fluenced by the presence of solutes. The solutes may ( 1 ) reduce the melt ing point or degree of supercooling, thereby reducing the freezing rate, or (2) by changing ice growth patterns increase the rate of the advancing ice front. The latter condition is expected to predominate. Irregular ice growth would entrap solute-rich liquid precluding migration of the solute


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into the bulk liquid. The concept of the planar growth pattern for ice and the assumption of a distinct boundary layer is idealized. Drost-Hansen (6) reports that surface orientation of the advancing ice front may extend 1000 A . or more into the liquid phase and be highly irregular. Mixing or turbulence may also affect dendritic growth. M i x i n g induces turbulence and a decrease i n the boundary layer thickness at the ice-liquid interface. The thinner the boundary layer, the shorter the time interval necessary for solute diffusion from the interface into the bulk liquid. The extent of incorporation i n the ice depends in part on the relative velocity of solute diffusivity and the advancing ice front formation. Therefore, once a critical mixing rate is reached, further turbu lence increase has no appreciable effect (Figures 2 and 3 ) . Dissolved inorganic solids have been shown to inhibit organic recov ery efficiency by freeze concentration from aqueous solution (4). Simi larly, increased salt content reduces cationic recovery (Figure 7 ) . A s ionic concentration increases at the interface, there is an increase i n tendency toward dendritic growth and associated entrapment, changes in surface tension and induced interfacial potential, and impedance of specific ionic migration. Movement of suspended and nondissociated materials ahead of the advancing ice front has been reported to be more efficient than for com pletely dissociated constituents (4, 14, 17). The previous statement may be valid for organics and to a degree for heavy metals, Figure 5, but it does not explain the observed separation of the alkali metals. The heavy metal cationic recovery is greater at increased p H but the alkali metal recovery is lower. The results of Figures 5 and 6 pertain only to recoveries measured when residual volumes were less than 50 m l . The occurrence of selective ion incorporation has precluded the use of an internal standard as an indicator of the overall recovery efficiency, a technique applied successfully to the concentration of organics (4). Be cause of this inability at the present time to quantify the efficiency of con centration, the application of the freeze concentration technique solely for the determination of trace inorganics is impractical. There are more precise methods available for inorganic concentration—e.g., evaporation or solvent extraction. Further refinement of the freeze concentration tech nique for cations may be achieved by washing the ice after transfer of the residual liquid. This point is being confirmed by current studies. Summary The study is a direct outgrowth of a continuing investigation of the freeze concentration of organic microcontaminants in water. The concen tration of inorganic constituents is being monitored b y atomic absorption


162

TRACE INORGANICS I N WATER

spectroscopy. The results w i l l contribute fundamental information eluci dating the parameters observed to affect organic concentration and permit development of a supplementary trace inorganic concentration scheme for water analysts. Waters of varying composition are being in vestigated. Specific observations include: (a) Selective ion incorporation in the ice has been observed in freezing from all aqueous matrices. The cations studied were F e , M n , C u , Zn, Pb, N i , C a , M g , and K . Selective incorporation precludes use of a cation as an internal standard for estimating overall recovery efficiency as is the case with complex mixtures of organic compounds. (b) Increasing mixing rate (or turbulence) increases cationic recov ery up to some limiting value for any given initial total dissolved solids content. Further increase in mixing has no effect on recovery efficiency. This is controlled by the cationic migration rate through the interfacial boundary layer ahead of the advancing ice front. The boundary layer thickness and hence, the required distance for diffusion and associated migration time interval depends on mixing rate. (c) The effect of initial p H on recovery efficiency depends on the nature of the cation. Alkali metals ( K , C a , and M g ) concentrate best at acid p H . Heavy metal cations (Pb, N i , and C u ) concentrate best at alkaline p H . ( d ) As initial total dissolved solids (specific conductance) increase, recovery of specific cations decreases. (e) N o variation in recovery efficiency of individual cations origi nally present in the concentration range of 0.1 to 10 mg./liter has been observed in single-stage freezes. For example, this was the case even in a complex mineralized water matrix of approximately 100 mg./liter total dissolved solids and 200 /^mhos/cm. conductivity. (f ) W i t h the experimental equipment and operating procedure being used, a physical limitation for single-stage volume concentration ratio was observed. The final volume of a single-stage freeze should be greater than 50 to 70 ml. regardless of initial volume to avoid marked decrease in cationic recovery. ( g ) A cascade of single-stage freeze concentration such as that used in achieving high organic concentration ratios has been shown to be advantageous in concentrating cations at trace and micro levels. Acknowledgment This paper is a contribution by the Environmental Sciences Fellow ship, Mellon Institute, Pittsburgh, Pennsylvania. The investigation was supported by D H E W ( N I H ) General Research Grant F R 558001-5. Literature Cited (1) (2) (3) (4)

ASTM Std. 23, 306 (1964). Baker, R. Α., J. Water Poll. Cont. Fed. 37, 1164 (1965). Baker, R. Α., Water Research 1, 61 (1967). Ibid. 1, 97 (1967).


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(5) Chalmers, B., E d g a r M a r b u r g Lecture, 64th A n n . Meeting A m . Soc. Test ing and Mater. (1961). (6) Drost-Hansen, W., Ind. Eng. Chem. 57, 18 (1965). (7) Gross, G . W., J. Geophys. Res. 70, 2291 (1965). (8) Heinmets, F., Trans. Faraday Soc. 58, 788 (1962). (9) Kobayashi, S., L e e , G. F., Anal. Chem. 36, 2197 (1964). (10) Lecointe, M., Rev. Gen. Thermique 4, 629 (1965). (11) Lodge, J. P . , Baker, M. L., Pierrard, M. J., J. Chem. Phys. 24, 716 (1956). (12) M e l i a , T . P . , J. Appl. Chem. 15, 345 (1965). (13) Parreira, H. C., E y d t , A. J., Nature 209, 33 (1965). (14) Parungo, F . P . , Lodge, J . P . , J. Atm. Sciences 22, 309 (1965). (15) Shapiro, J., Science 133, 2063 (1961). (16) Smith, G . H., Tasker, M. P . , Anal. Chim. Acta. 33, 559 (1965). (17) U h l m a n n , D. R., Chalmers, B . , Jackson, Κ. Α., J. Appl. Phys. 35, 2986 (1964). (18) W o r k m a n , E. J., Reynolds, S. E., Phys. Rev. 78, 254 (1950). RECEIVED

M a y 29,

1967.


Cationic Concentration by Freezing - Advances in Chemistry (ACS

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