Trace organic contaminant concentration by freezing—II: Inorganic aqueous solutions

Water Research, Pergamon Press 1967. Vol. 1, pp. 97-113. Printed in Great Britain.

TRACE ORGANIC CONTAMINANT CONCENTRATION BY FREEZING--II: INORGANIC AQUEOUS SOLUTIONS ROBERT A. BAKER Mellon Institute, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, U.S.A. (Received 7 November 1966) Abstract--The presence of dissolved inorganic salts in aqueous solution impairs recovery efficiency of organic solutes by a freeze concentration procedure shown to be effective in the absence of salt. This effect is observed in the recovery of acetophenone and various phenols. Although previous tests demonstrated that mixing rate is not critical in the recovery of trace solutes by freeze concentration of distilled water solutions, it is important when dissolved salts are present. Higher mixing rates increase organic solute recovery. A relationship exists between salt content, mixing rate during the freezing process and organic solute recovery efficiency. Adjustment of sample pH becomes a factor in organic solute recovery. This is increasingly important as volume concentration ratio increases. Recovery is greater from acidic solutions. Freeze concentration of a complex mixture of phenolic materials showed no selectivity. All components were equally concentrated. This suggests the addition of a known quantity of an organic material to field samples as an internal reference. The reference would permit correction of results for losses induced by salt, inadequate mixing, etc. No effect on recovery is observed when iron, calcium, magnesium or copper is added to dilute organic aqueous solutions in excess of theoretical maximum coordination concentrations. Organometallic complexes, if they form, do not alter freeze concentration recovery efficiency. The advantages of combined freeze concentration and gas-liquid chromatographic analyses over conventional analytical procedures for separating and identifying complex mixtures of trace organics at less than 1 mg/l initial concentration is demonstrated with a mineralized industrial waste water. INTRODUCTION

RECOVERYof#g/l and mg/1 concentrations of organic materials from aqueous solutions without chemical alteration or modification of relative distribution in complex mixtures is a requirement in many water pollution-related investigations. The commonly used concentration techniques, such as carbon adsorption-desorption, distillation and solvent extraction, do not always satisfy this requirement. Freeze concentration has been suggested as a potentially useful preanalytical procedure by SsnPmo (1961), KOBAYASHIand LEE (1964) and BAKER(1965). However, these earlier investigations were fragmentary. Chemical, physical and mechanical-geometric parameters were not adequately defined, and conflicting conclusions regarding recovery efficiency of various solutes arose from the first two papers. A systematic investigation of the process of freeze concentration of organic materials in aqueous solution is underway in this laboratory. The initial phase of the study was specifically concerned with organic-aqueous systems low in inorganic salt content. The results were described in the first paper o f this series, BAKER (1967b). The study involved p h e n o l a n d substituted phenols, volatile fatty acids a n d acetophenone. Single-stage a n d cascades o f singlestage freezing steps were used to recover quantitatively trace a m o u n t s o f these solutes. Mixing speed was f o u n d n o t to be a factor in recovery from distilled waters. Freezing 97

98

ROBERT A. BAKER

was accomplished in an ice-salt bath held at approximately -12°C. Complex mixtures of organics were concentrated without selectivity. All trace-organic components were recovered with equal efficiency at comparable volume ratios. Organic recovery decreased in efficiency in the same range of unfrozen residual liquid volume, regardless of starting volume. This is an experimental limitation of the system and technique employed. As residual liquid volume decreases, the liquid holdup in the flask and unwashed ice contributes an increasing percentage error when the concentrate is transferred. Thus, a limiting volume ratio exists per single-stage freeze concentration as a function of initial volume if total recovery is to be achieved. No effect on recovery efficiency was found for organic dissociation potential; molecular size or weight; or nature, location and number of substituent groups. OBJECTIVE

This paper summarizes the results of the second phase of the study, which is concerned with organic recovery efficiency from aqueous systems containing inorganic salts. Emphasis is placed on tap and industrial waste water. EXPERIMENTAL

Details of the freezing mechanism, equipment and experimental procedure are reported in the first paper, BAKER(1967b). Concentration occurs via selective crystallization of water accompanied by solute enrichment of the residual concentrate. If the process is interrupted before freeze completion and the residual concentrate analysed for solute, then the recovery efficiency may be calculated by comparing the chemical concentration ratio, CL2/CL. and the volume ratio, V~/V2: E = [(CL2 /CL~)/(V~/V2)] 100 per cent, where : CLI CL2 V1 /I2

= = = =

initial solute concentration in liquid (mg/l); final solute concentration in liquid (mg/1); initial sample volume (ml); final volume of concentrate (ml).

Concentration is accomplished in 1-1., round-bottom pyrex flasks rotated at controlled speeds of 0 to 260 rev/min while submerged in a mixture of crushed ice and salt at approximately - 12° C. A single-stage complete freezing of 200 ml requires approximately 20 min at 80 rev/min. The freezing process is interrupted at varying stages of completion; the concentrate is poured off, its volume is measured and the solute content analysed. The ice which forms on the flask walls is not washed. Analysis for organics is by direct-aqueous injection gas-liquid chromatography using flameionization detection, BAKER(1966, 1967a). No significant difference has been detected in recovery efficiency of the various organics tested. Consequently, most of the results of this phase of the study were obtained using m-cresol as the organic solute. Tests which involve the effect of specific cations (e.g. iron, copper, etc.) were monitored by atomic-absorption spectroscopy. Overall reproducibility of the method, including the freeze concentration and GLC analytical procedures, BAKER (1966), varies as a function of (1) organic content

Trace Organic Contaminant Concentration by Freezing--II

99

initially present in the sample, (2) the initial sample volume and (3) the volume ratio after concentration. Chromatographic peak area measurement may deviate from the mean by as much as 8 per cent at 1 mg/l concentration and 4 per cent at 10 mg/1 concentration for a typical phenolic material. However, typical deviation is of the order of 2 per cent at these lower levels. Analytical reproducibility is generally within 1 per cent at more concentrated organic levels; e.g. > 10 mg/1. Variation in measurement of residual liquid may be as much as 0.5 ml with the equipment and technique used. This error is attributed to holdup of concentrate on the unwashed ice and flask walls. An additional variance is caused by continued freezing of residual liquid between interruption of the run and pouring of the concentrate. A 0.5 ml volume difference represents a 10 per cent error at 5 ml. Despite these variances, the overall reproducibility is estimated to be of the order of 2--4 per cent. RESULTS The addition of inorganic salt to distilled water solutions of organic materials impairs recovery efficiency of the organic solute in subsequent freezing. This effect is illustrated in FIo. 1 for the case of sodium chloride addition to an aqueous solution of acetophenone. Acetophenone recovery decreases as the chloride ion increases from 0 to 100 mg/l (0 to 165 mg/1 NaC1). This particular example of freeze concentration 20O

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FrO. 1. Cascade freeze concentration of acetophenone in distilled water with salt present in two steps. Initial volume 200 ml for each of four single-stage freezes composited in Step One to approximately 200 ml; Step Two is freeze of Step One composite; ice-salt bath temperature = - 9 to -12°C; chloride added as sodium chloride; flask rotated at 80 rev/min. was accomplished by two-step cascade process made at 80 rev/min flask-rotation rate. Four 200 ml sample volumes were concentrated separately, approximately to 50 ml each, then the concentrates were combined and the 200 ml composite was refrozen

]00

ROBERTA. BAKER

as the second step. The acetophenone, initially at less than mg/l concentration, was recovered completely in the absence of salt up to a volume ratio of approximately 33: 1. However, total acetophenone recovery fell sharply when volume ratios exceeded 9 : 1 and 6 : 1 for initial chloride ion content of 10 and 100 mg/l, respectively. Chloride content of the ice and concentrate was not measured during these tests. Organic concentration from mineralized tap waters is less efficient than from distilled waters at comparable volume ratios and operating conditions. Tap waters from Pittsburgh and South Pittsburgh supplies were used to demonstrate this effect. These waters differ in source and often in composition of trace contaminants. The former is obtained from the Allegheny River and the latter from the Monongahela River. Typical chemical analyses of spot samples used during the study period are shown in TABLE 1. The recovery of 1.0 to 10.0 mg/1 m-cresol from tap waters was determined by single-stage freeze concentration at 80 rev/min ; approximately - 12°C ice-salt bath TABLE1. TAP WATERANALYSES

pH Conductivity ~mhos) Total solids (mg/l) Iron, Fe (mg/l) Manganese, Mn (rag/l) Calcium, Ca (mg/1) Magnesium, Mg (mg/l) Hardness, Ca CO3 (nag/l)

Pittsburgh tap water

South Pittsburgh tap water

9.0-9.4 (7.78)* (517)* 202-362 (231)* 0.05--0.10 0.05-0.20 27.2-49.6 4.9-8.7 90-162

8.4-8.8 (7.7-8.3)* (370)* 166-296 (203)* 0.03--0.07 0.01-0.04 24.0-31.2 -95-109

50:50 South Pittsburgh tap and distilled water

(7.5)* (260)*

* Bracketed values are specific spot analyses of laboratory test samples ; other values are watertreating plant analyses. temperature; and 200 ml initial sample volume. Recovery from Pittsburgh tap water (FIG. 2), South Pittsburgh tap water (FIG. 3), and a 50: 50 mixture of South Pittsburgh and distilled waters (FIG. 4) may be compared with distilled water recovery in FIG. 5. The distilled water data were obtained from Phase I, BAKER (1967b). Prediction of freeze-concentration efficiency on the basis of conductivity is not implied to be universally applicable. This relationship is fortuitous. Conductivity and, hence, ionic strength of most natural waters depends on the nature of the substances composing the mineral content, pH, etc. However, during the period of study the conductivity provided a measure of salinity ionization which correlated fairly well with recovery because other parameters did not vary appreciably. In Phase I of this study with distilled waters, it was determined that recovery efficiency was not dependent upon initial sample volume but, rather, upon a limiting final concentrate volume, regardless of initial sample size. Similar behavior occurred with the South Pittsburgh tap water, FIG. 3, and the 50:50 distilled-tap mixture, FIG. 4.

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FIG. 2. Eff~iency of m-cresol recovery in single-stage freeze concentration from Pittsburgh tap water as a function of volume concentration ratio. Temperature of ice-salt bath = - 11 to - 1 2 ° C ; flask rotated at 80 rev/min. Initially 200 ml of 1.0 mg/l m-cresol in 200 ml water; pH = 7.78; 517/tmhos; total dissolved solids = 231 mg/l.

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Trace Organic ContaminantConcentrationby Freezing--II

103

The flask-rotation rate (i.e. mixing) was found not to affect organic recovery from distilled water in Phase I at the ice-salt freezing temperature; however, the presence of inorganic salts markedly alters this observation. An interrelationship between salt content, flask-rotation rate and organic recovery exists. The effect of mixing is shown in FIG. 6 for the single-stage freeze concentration of m-cresol from tap waters of 336 to 363/~mhos conductivity. Efficiency of m-cresol recovery from this particular water by freezing at 260 rev/min coincides exactly with that in distilled water over the entire volume ratio range. However, there is loss of organic recovery efficiency at given volume ratios as mixing speed drops to 80 and 0 rev/min. This effect may be attributed to : (1) impairment of transfer of the organic solute because of salt concentration

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F]o. 6. Effect of mixing speed on single-stagefreeze concentration of South Pittsburgh tap water. Initial: volume = 200 ml; m-cresol = 1.0 mg/1; ice-salt bath temperature = - 9 to - 12°C; p H = 8.2 to 8.55; conductivity= 336 to 363 a r n h o s ; total dissolvedsolids -- 203 to 215 mg/l. at the interface and (2) the possible alteration of the crystalline growth of the ice induced by salt. In distilled water, freezing the water molecules in the liquid layers near the ice may already assume an ordering or arrangement comparable to ice prior to crystallization of the advancing ice front. Solutes, particularly electrolytes, may alter this ordering and, hence, ice growth. Mixing reduces the thickness of the interfacial boundary layer and the zone of possible ordering, but it also facilitates solute migration. Movement of the inorganic solutes away from the interface minimizes surface potential alteration. The use of a cascade or multiple-step freeze-concentration procedure was demonstrated to be feasible and advantageous for recovery of trace organics from distilled water solutions. Similarly, the advantages of combining increasing mixing and the cascade process for mineralized water concentrations are shown in FIG. 7. Step One of the cascade consisted of six individual freezes, each beginning with 200 ml. The composite was then refrozen in Step Two. For comparative purposes, FIG. 7 depicts the recovery of m-cresol in distilled water and from tap water at 80 and 260 rev/min flask-rotation rates. Previous results, BAKER(1967b), showed that m-cresol recovery from distilled water was equally efficient at all flask mixing speeds from 0 to 260

104

ROBERT A . BAKER

rev/min. This applied for both single-stage and cascade concentrations as long as a limiting final concentrate volume of approximately 30 ml was not exceeded. In tap water of 588/~mhos conductivity, however, efficiency at 260 rev/min, though better than 80 rev/min, is not as high as in distilled water at comparable volume ratios.

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FIG. 8. Effect of iron and pH on m-cresol recovery in single-stage freeze concentration from mineralized water. South Pittsburgh tap and distilled waters mixed 50" 50; initial volume = 200 ml; flask rotated at 80 rev/min; ice-salt bath temperature = - 9 to - 12°C; p H adjusted by NaOH and HCI; m-cresol = 5 mg/l; 5 mg/l Fe added as ferric chloride in solid points.

T A B L E 2 . E F F E C T OF IRON AND

pH

ON m-CRESOL RECOVERY IN SINGLE-STAGE FREEZE CONCENTRATION FROM MINERALIZED WATER ¢'1

Sample Run

pH

Ice

Conductivity (/tmhos)

pH

Liquid

Conductivity (pmhos)

pH

Conductivity (pmhos)

Conc. ratio Vol. (ml)

m-Cresol (mg/I)

28 15 42 17 37 13 38 12.5 28 14.0 9.5 33

31.1 47.9 23.4 40.1 22.2 43.1 26.2 57.0 28.6 45.4 50.5 24.7

Vol.

Chemical

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~ V," 0

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3.05

770

6.85

353

9.95

470

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605

9.97

665

3.98 3.85 4.50 4.62 4.98 5.60 3.98 3.70 5.35 5.30 5.52 5.28

134 301 60 93 115 193 111 323 135 238 417 178

2.42 2.45 5.20 5.50 5.79 6.28 2.81 2.42 6.09 6.10 6.00 5.90

3390 4620 1320 2450 1520 3110 3600 7790 2780 4370 4230 2510

7.15 13.3 4.76 11.8 5.41 15.4 5.26 16.0 7.15 14.3 21.0 6.07

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87 72 99 68 84 56 99 71 80 63 48 82

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ROBERT A. BAKER

The organic materials studied are all weak acids, except for acetophenone. The pH of the aqueous system will determine the degree of ionization of the organic acids, and indeed, the anionic form will predominate in more alkaline solution. The effect of pH of a 50:50 mixture of tap water and distilled water on recovery efficiency of m-cresol, p K ~ I0, is shown in TABLE2 and Fie. 8. The pH was adjusted with NaOH and HC1 to approximately 3, 7 and 10. These single-stage results indicate that recovery is enhanced at more acidic initial sample pH. This effect is not evident until the singlestage volume ratio exceeds about 5: 1. For a 200 ml sample volume, this means that concentrate volume must be less than approximately 40 ml. This is the concentration zone where efficiency impairment begins even in distilled water solutions because of the aforementioned experimental and mechanical limitations. The data of FIG. 8 are conveniently depicted as linear regressions, but previous results indicate that as freezing approaches completion the efficiency drops significantly. Despite the variability expected as freezing approaches completion, there is no question that m-cresol recovery was more efficient in the acidic state, This same effect has been observed in other mineralized aqueous samples and is not a spot observation. COMPLEXES The chemical states in which metals in solution are found reflect the relation between the acid-base, redox, and complex-forming properties of these solutes on the one hand, and the pH, potential, and ligand concentration of the solution of the other. In this study, the possible complexes of the cations and organic ligands formed were of interest. A review and summary of Fe(III) and phenolic complexing behaviour was presented by GOR~ and NEWMAN(1964). The stability constants of many cationligand complexes were determined during the course of this extensive research. A specific form of complex is the chelate. A chelate typically involves an organic ring structure binding a metallic atom at its center by both coordinate and polar valence bonds. The chelates exhibit properties quite different from uncomplexed metallic ions. The effect these complexes might have on freeze-concentration efficiency, if present, was unknown. In natural, mineralized waters, chemical equilibrium relationships determine the ionic forms which exist. For example, the form of iron in solution is quite specific at various Eh and pH values. This has been described by HEM and CROPPER (1959). Typically, natural waters maybe expected to contain less than 0.01 mg/l iron between pH 5 and 8 and Eh between 0.3 and 0.5 V. At pH 5 and Eh 0.3 V, Fe 2+ content may exceed 100 mg/1. In aerated waters of pH greater than 5, iron in excess of 0.01 rag/1 can only be present as oxide or hydroxide suspensions. For natural water systems, it is necessary to consider inorganic complex ions which, at low pH, affect behavior of ferric iron. In highly acid aqueous solution, chloride complexes are important. At pH 5 to 8, they do not seem to be significant. Other inorganic ions may be involved in complex formation, such as fluoride, phosphate, sulfate and carbonate. The effect of iron on freeze concentration was examined by adding 5 rag/1 iron as ferric chloride and 5 rag/1 m-cresol to the tap water of TABLE2 and FIG. 8. If only the coordination number of 6 for iron is considered, then it was present in excess of stoichiometric requirement. However, the controlling factor is the formation constant of the complex under consideration vs. the concentration of the complex-forming

Trace Organic Contaminant Concentration by Freezing--II

107

ligand. No effect was evident at pH 3, 7 or 10. Metallic-ligand or metallic-chloride complexes, if formed, did not influence organic separation. To be sure the aforementioned operation was not affected by the gas--liquid chromatographic procedure, a series of analyses were made. It was found that no error was induced in m-cresol analysis by iron, copper, magnesium or calcium in distilled or tap waters. These tests were made at varying cation: organic ratios and at pI'I 2.75 to 10. The presence of other inorganic cations in tap water may perhaps prevent iron from affecting cresol recovery. A test of this possibility was made with a laboratoryprepared solution of known composition with and without iron supplement. This solution was buffered by 10-3M KH2PO4 plus 10-3M K2HPO4 at pH 7 in distilled water and contained 5 rag/1 m-cresol. Single-stage freeze concentrations were made at 260 rev/min of this solution with and without the addition of 5.2 mg/1 iron as ferric chloride. A fine, white precipitate of ferric phosphate formed in the iron-containing solution, but m-cresol was complete in both cases at a volume concentration of 4.44: 1. Ferric recovery in the liquid concentrate was 89 per cent efficient. Filtration of the sample containing the ferric phosphate precipitate through a 0.45/~ millipore filter produced a filtrate measured at 0.22 mg/1 iron indicating the relative size of the precipitate. Initial pH of the samples used in these tests was 6.04 with a conductivity of 356/~mhos. Freeze concentrations of laboratory-prepared aqueous solutions weakly buffered at 9.6 pH, with and without supplemental salt content, were made with 0 and 5 mg/1 iron, TABLE3. There is no effect on concentration efficiency for iron at the alkaline pH, nor is there an apparent organic ionization effect since the initial pH of 9.6 approximates the pK value of 10 for m-cresol. There is an effect between the 0 to 5000 KC1 supplemented solutions. A marked drop in cresol recovery was determined in the more salted solution. TABLE 3. EFFECT o f IRON ON FR-CRESOL RECOVERY BY SINGLE-STAGE FREEZE CONCENTRATION

Sample

KCI (mg/l)

0

Conductivity (/tmhos)

79

pH

Volume Fe 3 ÷ ratio, (mg/l) 1/1/V2 2.6 4.8 10.3

100 93 83

2.6 4.8 10.3

99 91 86

0

2.5 4.3

76 59

5

2.5 4.3

74 61

9.6

1 0 a M K2HPO4 plus 2.4 x 10-4M K O H buffer in distilled water

5000

8240

m-Cresol recovery efficiency (~)

9.6

Initial volume = 200 ml ; m-cresol = 1.0 mg/l ; flask rotated at 260 rev/min.

108

ROBERTA. BAKER

The effect of copper, calcium and magnesium ions on recovery of 10 mg/l m-cresol from aqueous solution was examined at p H 3 and 10. The solutions were buffered by (a) 10-aM KH2PO,t plus 1.21 x 10-3M H3PO4 and (b) 10-3M K2 HPO4 plus 1.65 x 10-3M KOH. These buffers have equal salt loadings and common ions. In separate tests, the effect on organic recovery efficiency for each of these cations at both pH levels was studied. The cations were added at 25 mg/l concentrations as the chloride salts. Singe-stage freeze concentrations were made at - 12°C ice-salt bath temperature and with 260 rev/min flask rotation rate. Despite the precipitate formation at pH 10, the results showed no significant difference in m-cresol recovery at either pH 3 or 10 levels, or for the addition of any of the cations added. WASTE W A T E R The effluent from a blast furnace washer water settler was analysed directly by the conventional colorimetric procedure and by gas-liquid chromatography after freeze concentration. This is an interesting analytical comparison because the relative advantages and shortcomings of both methods are clearly demonstrated. TABLE 4 presents an analysis of this waste water. The phenol content was measured as 715/~g/l by the STANDARD METHODS (1965) distillation, 4-aminoantipyrene procedure. A preliminary G L C analysis indicated the presence of peaks at the elution intervals of TABLE 4. BLAST FURNACE WASHER WATER ANALYSES

pH* Conductivity* Suspended Solids Iron* Calcium* Magnesium* Alkalinity* Ammonia Cyanide PhenolJ" Phenolics~ phenol, o-cresol In- and p-cresol

8.2 2880 ~mhos/cm) 256 (rag/l) 0.06 (rag/l) Fe 70.6 (mg/l) Ca 24.6 (mg/l) Mg 450 (rag/l) CaCO3 100 (rag/l) N 0.39 (rag/l) CN 715 (/,g/l) phenol 606 (gg/l) phenol 492 (/~g/1)

* Analyses after filtration. t By distillation and 4-aminoantipyrene. :[:By gas-liquid chromatography after 5.8 : 1 freeze concentration. phenol and m-cresol and for several higher-boiling materials. No additional peaks were indicated for faster-eluting organics. The waste water is not chlorinated so 12 mg/1 o-chlorophenol was added as an internal reference. This compound elutes ahead o f the phenol with the F F A P chromatographic column used. Its use permitted an estimate o f freeze-concentration efficiency. Single-stage and cascade freeze concentrations of the phenolic organic materials being studied have shown that recovery efficiency does not vary among components of a mixture. This is true for distilled water solutions, BAKER (1967b), and in mineralized waters, TABLE 5.

a

TABLE 5. SINGLE-STAGE FREEZE CONCENTRATION OF TERNARY PHENOLIC MIXTURE IN TAP WATER

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Ice

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pH

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1

48

6.28

4220

2

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Vol.

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124

8.33

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ROBERTA. BAKER

A 250 ml sample of washer water was concentrated at 80 rev/min to 43 ml or a volume ratio of 5.8 : 1. The reference o-chlorophenol was concentrated 3.44 times for a 59.5 per cent efficiency. A peak occurring at the elution interval for phenol and o-cresol was measured to be equivalent to 606 #g/l phenol after the 0.595 concentration efficiency factor was applied. A second peak at the m- and p-cresol elution time was calculated to be equivalent to 492 #g/l of these organics. Details of the GLC procedure, including calibration factors, elution intervals and Comparison of response to various phenolics between G L C and 4-aminoantipyrene, have been described, BAKER (1966). A third peak with an elution interval approximately 1.9 times that of phenol 8),(

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t

15

t

Time, Min

10

r

o

FxG. 9. Gas-liquid chromatographic analysis of blast furnace gas washer water. Aerograph Model 204-1B with time-ionization detection; column 5 ft x ~-in. stainless steel, 10 per cent FFAP on 40/60 chromsorb T; temperature column = 157°(2; temperature injector = 205°C; nitrogen carder gas at 25 ml/min; hydrogen at 25 ml/min; electrometer range = 0.1 ; attenuation = 1X; 1 mV recorder at 30 in/hr chart speed; 5/zl sample containing 12 mg/l o-chlorophenol (A) as internal standard. Peak 03) is phenol and/or o-cresol; ((2) is m- and/or p-cresol. (D) is not identified. was obtained. This higher boiling material could be estimated at 128 #g/1 based on an assumed calibration value equivalent to half that of phenol. This is a conservative estimate based on experience with phenolic materials eluting at this relative time. However, for reporting purposes, only the first two peaks are specifically identified. The combined total of 1098 #g/l by G L C is considerably higher than the 715 pg/l measured by 4-aminoantipyrene. The colorimetric procedure only determines 0.38 of m-cresol and none of the p-cresol based on calibration with phenol. The G L C procedure combined with freeze concentration permits more specific component identification and measures more of the total organics present in this waste water. Supplemental mass or infrared spectrographic analyses of the peaks eluting from a parallel chromatographic column after splitter injection would further define the trace organics being separated. The conductivity of the liquid concentrate after freezing was measured as 8620 /amhos and of the melted ice as 1500 #mhos. Respective p H values were 7.05 and 6.70,

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demonstrating the effect of atmospheric CO2 adsorption on the aqueous system which, in this case, was originally at pH 8.2. The pH modification by CO2 precludes expectation of precise conductivity material balance and, hence, calculation of inorganic concentration efficiency as for the organic material. The state of dissociation of some inorganic salts will vary with the induced pH. Despite this consideration, the conductivity balance is accurate to about 5 per cent and conductivity concentration efficiency is measured at 51.5 per cent. This compares favorably with the 59.5 per cent efficiency for the o-chlorophenol concentration. DISCUSSION

It is evident from these results that dissolved inorganic salts reduce the efficiency of recovery from aqueous solution of trace-organic solutes at given parametric conditions. Explanation of the phenomena observed is only partially possible at this time. Some clarification may be derived from previous studies of aqueous solutions of inorganic salts. Salt effects on freezing have been examined for centuries. Despite this long span of interest, the mechanisms; interrelationship between physical and chemical parameters; and, particularly, the behavior of organics in complex mixtures of inorganic and organic solutes in water on freezing are poorly understood. Critical examination of interfacial effects by DROST-HANSEN(1965), of crystal nuclation by M~LIA (1965) and of inorganic charge separation and electrical potential on freezing by GROSS (1965) have been the most recent summaries of knowledge. Gross, in particular, oriented his discussion and experimental efforts in terms of the WORr,MANREYNOLDS (1950) effect. This is now the classical foundation for ionic transfer proeesses at the ice-solution interface. From previously published research results and the experimental observations, the effect of the inorganic salts on organic recovery is deduced to involve modification of ice structuring with disruption of uniform ordering of the surface and, hence, enhancement of the entrapment of unfrozen liquid; and physical impeding of organic solute migration at the interface into the bulk liquid concentrate. Salts are often added to aqueous systems to increase organic separation in solventextraction procedures. If salt addition decreases solubility of the solute, a process known as salting-out occurs. Recovery is usually enhanced, particularly with non-electrolytes. However, organic solute solubility may be increased by salt addition and, in this ease, salting-in occurs. In these studies, salting-out and salting-in effects are not considered to be a factor influencing organic solute recovery. The rate of freezing is known to affect freeze-concentration efficiency; e.g. SCHILDKNECnX and SCHLEGMILCa(1963) and others have demonstrated this effect. In these studies, an ice-salt bath temperature of approximately - 12°C was maintained with variation limited to temperatures up to - 9°C. Thus, freezing rate is solely a function of mixing rate. Naturally occurring waters are buffered to varying degrees by carbonate-bicarbonate-CO2. The acid pH shift during the freezing process induced by atmosphere CO2 solution was observed in all tests. It was, of course, much more pronounced in the first phase of this study with unbuffered distilled waters, BAKER(1967b). A nitrogen atmosphere over the freezing surface to minimize CO2 uptake failed to alter organic recovery. WORK_~_AN(1954) has reported that ice formation is sensitive to

112

ROBERT A. BAKER

impurities including CO2. If it is an effect on organic recovery in these studies, it too is masked by stronger effects. The conductivity of the sample, ice and concentrate measured during these tests, TABLES2 and 5, served as controls. Ionic strength varies with pH and, hence, CO2 uptake, precluding precise conductivity balances. RESULTS AND

CONCLUSIONS

The separation of trace quantities ~g/l to mg/l) of organic materials in aqueous solution in the presence of dissolved inorganic salts is reported. Single-stage freeze concentrations were made to study the parameters involved in organic solute recovery. (1) Addition of salts to distilled water solutions resulted in reduced efficiency of freeze-concentration of organic solutes. For example, 16.5 mg/l sodium chloride in an unbuffered aqueous solution of < 1 mg/1 acetophenone, a non-electrolyte, reduced single-stage freeze concentration recovery efficiency at volume ratios greater than 9 : 1 volume ratio, dropped to 74 per cent at 30. 8 volume ratio. (2) Recovery of 1.0-10.0 mg/1 m-cresol from tap waters of varying origin by freezing indicated that efficiency decreased as total dissolved solids increased for comparable operating conditions and at selected volume ratios. (3) Freeze-concentration efficiency of organic solutes is increased with increased mixing when inorganic solutes are present. This mixing effect was not observed in distilled water and the absence of inorganic solutes. An interrelationship between salt content, mixing rate and organic solute efficiency exists. (4) A pH effect was observed for the recovery of m-cresol from mineralized tap water. Single-stage freeze-concentration efficiency increased as pH was decreased from 10 to 3 at volume ratios exceeding 5: I. (5) Iron, copper, magnesium and calcium were tested in tap and laboratoryprepared buffered waters for possible cation-ligand complex effect on freeze-concentration efficiency of organic solutes between pH 3 and 10. No effect was evident. (6) Recovery of o-chlorophenol, phenol and m-cresol from a mineralized tap water containing 472 mg/1 total dissolved solids showed that the individual components were not selectively separated. The use of an organic component as an internal reference to indicate overall freeze-concentration efficiency in field applications is suggested. The addition of an internal reference is expected to be especially helpful in measurement of very low levels,/~g/1, of organic solutes requiring cascades of singlestage freezes to reach minimum detectability levels of analytical devices. (7) Separation and identification of complex mixtures of trace organics in aqueous solution by a combination of freeze concentration and gas chromatography was shown to be advantageous over conventional non-specific colorimetric analyses. The major organic components of a blast furnace washer water were identified at concentrations originally less than 1 mg/l by the freeze-GLC procedure. The effect of inorganic salt concentration, pH, mixing rate and organic solute dissociation on freeze concentration efficiency from synthetic, known-buffered solutions is under investigation and will be described in a subsequent paper.

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Acknowledgements--This study was supported by the Environmental Sciences Fellowship, Mellon Institute, and is abstracted in part from a thesis submitted in partial fulfillment of the requirements of D.Sc. at the University of Pittsburgh.

REFERENCES BAKER R. A. (1965) Microchemical contaminants by freeze concentration and gas chromatography. J. Wat. Pollut. Control Fed. 37, 1164-1170. BAKERR. A. (1966) Phenolic analyses by direct aqueous injection gas chromatography. J. Am. Wat. Wks Ass. 58, 751-760. BAKER R. A. (1967a) Volatile fatty acids in aqueous solution by gas-liquid chromatography. J. Gas Chromat. In press. BAKERR. A. (1967b) Trace organic contaminant concentration by freezing--L Low inorganic aqueous solutions. Water Res. 1, 61-77. DROST-HANSENW. (1965) Aqueous interfaces. Proc. Syrup. btterfaees, Am. Chem. Soe. GORE P. H. and NEWMANP. J. (1964) Quantitative aspects of the color reaction between iron (III) and phenols. Analyt. ehim. Aeta. 31, 111-120. GRoss G. W. (1965) The Workman-Reynolds effect and ionic transfer processes at the ice-solution interface. J. geophys. Res. 70, 2291-2300. HEM J. E. and CROPPER W. H. (1959) Survey of ferric-ferrous chemical equilibria and redox potentials : U.S. Geological Survey Water-Supply Paper 149-A. KORAYASIJIS. and LEE G. F. (1964) Freeze concentration of dilute aqueous solutions. Anah,t. Chem. 36, 2197-2198. MEL1A T. P. (1965) Crystal nucleation from aqueous solution. J. appl. Chem. 15, 345-357. SC~JILDKNECnTH. and S¢,LEGEMILOJ F. (1963) Normales erstarren zur anreicherung und reinigung organischer und anorganischer verbindungen. Chemie-lngr-Tech. 35, 637-640. SHAPIROJ. (1961) Freezing out, a safe technique for concentration of dilute solutions. Science, N. Y. 133, 2063-2064. STANDARD METHODSFOR THE EXAMINATION OF WATER AND WASTE WATER (1965) American Public Health Association. 12th ed. New York. WORKMANE. J. (1954) On geochemical effects of freezing. Science, N.Y. 119, 73. WORKMAN E. J. and REYNOLDS S. E. (1950) Electrical phenomena occurring during the freezing of dilute aqueous solutions and their possible relationship to thunderstorm activity. Phys. Rev. 78, 254-259.

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