Reverse osmosis rejection of heavy metal cations

REVERSE-

OSM

515 REJECPIION

OF HEAVY

METAL

rejection of various heavy mttaf &!&de salts by ceflulosc osis membtancs is described. A series of rn~rn~r~~~~ exhi~~t~~g s oi from 26 ts 917~ is employed. ~l~t~vj~ batter for divalc~t metats subsets that jou-~va~cr rather than ion-membrrrne interactions are the controlling factors in cation rejections by dlulose acetate membranes, The partiat mofal free energiers of ies of the ions in s~~~t~o~ ~d~~atc~y represent these The selcctivc

acetate reverse 0s

controlling

factors.

The ~bi~~~yof membranes ta setcctively tejes=t e&ionic species f&m ~~~~~u~ solution is a phenomena which is of vital importance in biological systems (1). Awareness of these phenomena has subzxquentty assisted in the devetopment and study of various ion sektive electrodes 42) whose rn~ha~jsm ~~~~~3t~~~ involves semipermeable membranes which exhibit very specific sekctivities for different ions, More recently, the ~rnerge~~ of reverse osmosis has focussed further attention on the removal ofchargtrd (and uncharged species from safution by se-s. Desalination of brackish waters and sea waters were perhaps the original h a variety of subsequent ~mpo~a~t tions of reverse ~srn~~i~~ of e uents which may applications have emerged- Amon, a these is the treatment ificz2nt quantities of heavy metal ions in solution.. s~~~rat~on are rather i~corn~~~tc~y d~~~~ genetat selection criteria are proposed to at present_ Noncthetess, reason govern the ~hav~o~r of a given c f Paiute under must conditions. Such g~~~~~ ~~~jvjti~ are based on ~p~ro~~~atc therm namic criteria and may further be supported by fundamental physicochemical properks and interactions for each memb~n~so~utj~~ system. Lt is ~~upos~ in the present study to examine some . Pnscnt

addict:

Tctnicrant Limited,

10 Carson

Street,

Toronto, Chtaris, Canada

H. KIRK

of the general

thermodynamic

criteria

which

process and to present details of some specifically influence the selective removal

operate

during

JOHNSTON

the reverse

osmosis

of the physicochemical criteria which of heavy metal salts by cellulose acetate

reverse osmosis membranes. Definite patterns in selectivity have been observed in the study of ion rejection by organic, inorganic, charged, and uncharged petmselective memb&es (I-12). Such selectivities have been most rigorously characterized for the specific ion electrodes where removals of the alkalis aod alkaline earths have been studied

in detaii (I, 2). but distinctive

sequences

have also been observed

removals by reverse osmosis membranes (3-12). Loeb (3) proposed that rejection decreased unh$&ated ion and Michnels et al. (4) suggested

with

increasing

for cation size

of the

that rejection increased with increasing hydrated rarllus. Choi and Bennion (5) and Re and Bennion (6) held that hydrated radius controlled transport rates for small ions and crystallographic radius controlled these rates for larger ions. Glueckauf (7,8) and Bean (9) predicted decreased ion rejection witi decrease in unhydrated radii. Such contradictory views among these authors are similar to those initially proposed to explain the observed selectivities of ion exchange membranes. Cellulose acetate membranes are similar to ion exchange electrodes in certain aspects since the cellulosic films behave as if the surface has a slight negative charge (I, 13). Theories proposed to explain membrane selectivity on the basis of size and charge effects can qualitatively be .applied

to either membrane

type.

The separation of alkali cations by specific ion electrodes was experimentaliy of membranes, the most illustrative of which are the aluminosilicates such as ultramarine (14) and permutit (IS) which exhibited completely opposite selectivities for alkali cations in aqueous solution: achieved

with a variety

ultramarine membrane selectivity pattern I,, Li+ > Na+ > K+ > Rb* > Cs+ permutit Csf

>

membraoe Rb+

selectivity

> K+ > Na+

pattern

XI,

> Li+

Ion size was initially cited as the criterion for each of these observed rejection sequences. Pattern I,, was considered to depend on the non-hydrated ionic radii since the closeness with which the nqo-hydrated ions could approach the negatively charged membrane surface would determine the strength of the controlling electrostatic interactions. The smallest ion (Li+) could get nearest to the surfaceand would therefore be bound the tightest since the opposite charges on the cation and the membrane would be in the closest proximie to each other. The largest unhydrated ion (Cs’) would have the greatest distance between centres of charge and would,

there-fore, be ihe weakest bound cation in the series.

REVERSE

OSMOSIS

REJECI-ION

OF HEAVY

MEI-AL

207

CATIONS

In the case of the permutit membrane (pattern XI,), the inverted order of selectivity was rationalized on the basis of the apparent hydrated size of the ions. The smaller the unhydratcd

ion,

the greater

was

its extent

of

hydration_

thereby

effectively reversing the order of size between the hydrated and unhydrated species. Cesium, which had the smallest hydrated size, was most strongly attracted to the membrane. Conversely, the largest hydrated ion (Li+) was the least attracted to the negative aluminosilicate surface. If only these two patterns of selectivity were observable for all membranes rejecting the alkali cations, some support for this theory ma) have developed. However, other aluminoriiicate membranes exhibited seemingly anomalous selectivity patterns (16) which could not be ixoncihd to the simple conc_ept of ior_ic size. Thus, the apparent dependence of membrane rejection ability on a fundamental solute parameter does not define a general selection criteria unless that dependency is common for all membrane materials. Despite observed variability in the rejection sequence for the five alkali cations with different membranes, it was experimentally discovered that of the 120 possible sequences, only eIeven in fact were observed to occur (17). These sequences followed certain quantitative patterns (18) and the resulting selectivity isotherms suggested that some fundamental criterion for alkali cation rejection was in fact controlling the membrane selectivity processes. Attempts to explain these isotherms on the basis of hydration

order (15) or relative polarizabilities

of

the ion exchange site (19) were unsatisfactory. The first successful interpretation of the alkali cation membrane selectivity throughout the eleven rejection sequences was proposed by Eisenmann (I 7.21) on the basis of competingcoulombic attractive forces between the membrane and the water for the ionic species. In general terms, this criteria for membrane selectivity is dependent upon the difference in free energies of interaction of the solute species with the bulk solvent and of the solute species with the membrane surface. For the alkali cations, such free energies of interaction are mainly electrostatic in nature (I 7, 21). LJnder equilibrium conditions, the standard free energy difference between solutesolvent (A P”“) and solute-membrane (A Fmemb) interactions can be expressed in general terms as:

(1) where a and b refer to two different solute species being selectively separated. For ions in aqueous AF”

=

solution,

(F:d

-

this equation

F,byd) + (j$yb

has the form: _

pyb)

(2)

where @tYd - PEY”)is the difference on the partial molal free energies of hydration the ions and (Freb - Fremb) is the difference in their ionic free energies in the membrane phase. JZq.(2) is a general thermodynamic equation which is dependent only on the differences in the ion-water and ion-membrane interaction energies. of

208

H. KIRK JONHSI-ON

No mechanism is specified for such interactions, to apply equally

well to reverse

osmosis

thereby enabling this relationship or specific ion membranes.

In the case of strong membrane-ion interactions, the (emb

- FFmb) term

will exceed the hydration energy difference and the ionic affinity for the membrane (rejection ability) will decrease with increase in ionic crystal radius (alkali cation selectivity patrem I,). Conversely, when the ionic hydration energies predominate in Eq. 2, selectivity pattern XI* will result and rejection ability will decrease with increase in hydrated size. The other nine transition selectivity patterns (27) can be predicted from Eq. 2 in conjunction with thermodynamic or coulombic approximations regarding the nature aud magnitude of the cation--membrane interactions (17, 20). The alkaline earth cations have been observed in biological (22) and in ion exchange membranes to exhibit selectivity sequences similar to the alkali cations (21-25). A series of seven selectivity sequences have been found:

~a+~ >

>

> Ca+2

Ca+2 > Sr+’

> Mg+l

Ba+2 Cac2

> BaC2

> Sr”

> Mg+’ > Mg+’

Srf2

Ca+2

>

E~I+~

>

ca+2

>

Mg+2

>

ca+l

>

Mg”

Mgi2 Ba+2 > Sr+l

> ST+~ > Sr+’ > I%+2

Mg+’

> Ca+2

> Sr’2

> Ba”

I AE IL IhE Iv,,

V AE %E

VILE

When the free energy of hydration dominates equation 2, selectivity sequence IAE occurs, but when the ion-membrane interactions are stronger, the ion with the smallest ionic radius (Mg+2) is perferentially rejected. These divalent cation selectivity patterns are pH dependent and a shift occurs from sequence VIIAE to sequence I,, (i.e. strong field to weak field) with decrease in pH (25). For divalent cations other than the alkaline earths, coulombic forces have proven insufficient io explain observed selectivities (I, 2f). The necessary influence of noncoulombic

forces

becomes

more apparent

for these cations

and difficulties

result in attempting to justify observed behavior on a theoretically appropriate basis. The ability of a reverse osmosis membrane to selectively reject constituents from aqueous solutions is dependent on a variety of physical and chemical characteristics of both the polymer film and the solutes being separated. Some of this selectivity is a consequence of the specific nature of the polymer-solution interactions that must necessarily occur during the rejgtion process. Because of the number of variables involved and the experimental and theoretical difficulties presented in attempting to define and understand the criteria for rejection, few general conc!usions can be drawn .on the specific requirements neceSSary for the successful removal of constituents by a- reverse osmosis membrane without

REVERSE OSMOsIS REIECTION

initially undertaking involved.

OF HEAVY

a preliminary

METAL CATIONS

experimental

examination

209 of the class of solute

Consequently, the approach taken in the present study was to examine the alkaline earth cations as well as numerous other divalent cations from the fourth period of the Periodic Table. The general term “heavy metals” if often applied to these materials. A few monovalent and trivalent cations were also examined in order to provide additional general information. COMPARATIVE

REMOVAL EFFIClFh’CIES Of HEAVY METAL SALTS BY REVERSE OSMOSIS

The reverse osmosis membranes employed in this study were prebared in our laboratories from a cellulose acetate (Eastman 398-3) having a viscosity average molecular weight (26) of 27350 and a mecln degree of substitution of 2.45 (27). The composition of the casting solution in weight percent was: cellulose acetate 17.0; acetone 69.2; magnesium perchlorate 1.45; and water 12.35. This formulation has been employed by Kunst and Sourirajan (28) to reproducibly prepare high flux membranes when the relevant casting conditions are appropriately controlled. Membranes were cast on l/4 inch thick 8 x IO inch Pyrex plates by drawing the solution across the surface with a l/2 inch diameter Pyrex rod. Thickness of the membrane was controlled by the appropriate choice of the runner material (electric tape) fastened to the parallel outer edges of the casting plate. The membranes prepared in this manner had a uniform thickness of 0. I2 mm. Because of the sensitivity of this particular casting formulation to variations in temperature and humidity (28), all relevant parameters were carefully controlled through the use of a specially designed casting room. The temperature of the glass rod, the casting plate and the casting solution was 10°C. The atmosphere was maintained at 30°C. as was the surface on which the casting plate was placed during membrane fabrication. Relative humidity was 65 %. Subsequent to casting, solvent was allowed to evaporate from the surface of the membrane for sixty seconds. The membrane on the glass plate was thtrl immersed for several hours in a I-2°C gelation bath of distilled water. Membranes were cut to the size required for test purposes and each was shrunk for ten minutes in a water bath at a specified temperature. After shrinkage, the membranes were mounted in a static test cell (Fig. I) and pressurized in drstilled water at 300 psi for several hours. The stainless steel test cell had a sample capacity of approximately 220 ml. Pressure was supplied by a cylinder of compressed nitrogen gas and surface turbulence was controlled by a magnetically driven variable speed stirrer. Membranes were allowed to relax overnight and were tested the following day for their solute rejection ability with a reference solution of 3500 ppm NaCl under standard test conditions of 250 psi at a cell stirring speed of 1055 r-pm. Sodium chloride concentrations in the permeate

Fig_ 2, Shrinkage: rempcrature

prcsfitr: of oelluk~~ acetate mcrw

am&s

memlxa~-

REVERSE OSStOStS RElECTION

adopted

MErAL

21 t

CATlOSS

wherein the first 5 ml of permeate was discarded and the following

was colkcted manner,

OF HEAVY

and analyzed

a feed solution

to determine with

an initial

NaCI content

in the permeate.

concentration

of 3500 ppm

10 ml in this

would

be

concentrated to at most 3750 ppm, an increase of 7.1 T/,_ By uniformly employing the -mean feed” concentration in all calculations of separation efficiency, a consistent and reproducible measure of membrane selectivity could be determined_ Removal

efficiencies of individual

heavy metal salts were evaluated

using

initial feed solutions

of from f-1000 p?m of the specific metal and subjecting them to the reverse osmosis process under the standard conditions defined above. Comparative removal efficiencies of different salts were obtarned using solutions containing JO ppm of the individual metal. Metal concentrations in the permeate and the mean feed were determined by standard atomic absorption techniques (29). All atomic absorption analyses were performed immediately following sample collection and no acidification of the solutions was necessary, except for Be and Sn which were acidified with one drop of HNO, (cone) per 10 ml of sample and were analyzed seven days after collection. Solute separation, S, was determined

from the relationship: S = mean solute concentration in feed (ppm) - solute concentration -_-_..-_._. --_.-_--- --.--.-----------. mean solute concentration in feed (ppm)

in permeate (ppm) -_.____ -.-x 100%

A series of membranes exhibiting NaCl rejections of 91. 90, 86, 68, 66, 60 in the course of this study. These membranes were and 26:; were employed defined according to their sodium chloride rejection levers as N&&t, NaCl,e, NaC& NaCIe, and NaCl,,. All experiments were carried out NaCl,.. NaCI,,, at room temperature (23-25’C) and any variation in permeation rate with temperature was adjusted to a reference temperature of 25°C on the basis of the relative viscosities of water at the various temperatures. Solute rejections arc reported as they were observed. All calculations and linear least squares optimizations were performed on a Hewlett Packard Model 9830A computer. Resuits and discussion (i)

Membrane

consistency and definitiotl

in membrane performance over the duration of these tests was assessed on the basis of daily determinations of the NaCl rejection ability of the Iilm under standard conditions. The results summarized in Table I show the uniformity which was maintained. There was a small decrease in rejection ability with time for the NaClr6 membrane, possibly due to a slight hydrolysis of the acetate ester on the membrane surface. The rate of hydrolysis would be expected to increase in proportion to the flux across the polymer surface. The standard Variability

H. KIRK JOHNSTON

212

deviation (a) and standard error of the mean (S,) of the NaCl rejection abilities with time were evaluated for the Nat& membrane and exhibited values of 1.37 and 0.306 reqzctively. Other values are summarized in Table I. It has been reported that the concentration of heavy metal feed solutions exerts “a signif;canr effect on percent rejection for all metals” (30). However, no such behavior was readily apparent in the present study. As Table II indicates, TABLE

I

VARlAFilLlTY _ ___.

*3perafing

OF MEMBRAXE --.-__-_.-._

PERFORMANCE -.

Membrane ____~____._

NUClSl

Rtfec-

_--

FlfLdb~ .-. ..---.

1

91.6 90.7 89.9 90.8 91.2 91.4 91.6 91.3 91.0 90.6 91.4 91.0 91.0 90.9 91.4 89.7

---

TIME

_.-_ _._. _.

_...__ _____ _._._ .._ -_

-----

fief@

2 3 4 5 6 9. 10 11 12 13 17 18 19 20 21 24

-___

WITH

NaCifdj _____..

68.4 68.5 67.8 68.0 68.2 68.5 68.6 68.8 ::; 68.6 66.2 66.7 66.7 67.4 67.1 67.0 663 67.4 65.3 67.7 65.9 67.5

;: 41 42 45

91 .o 2:: 91.1 92.4 919

RIG,

91.1

22.5

-_-

FIUX Rejeclion ._-_ _- _..--. -.-

20.5 22.2 21.6 224 22.3 23.0 22.8 22.8 22.6 22.5 22.6 22.7 ‘27 22.7 22.6 22.4 23.0 22.6 21.9 22.7 228 22s

_ ..-

55.4 54.5 53.6 53.5 54.2 54.5 54.2 53.9 53.7

._.-._-

-.._-

NaCh -_ -

Rejec-

rim _-.-_

--.

--

--...-.

.

-_

..___

-.-Flux

NaCld -._-__-___RejecFIUX

-_-.

lion ________..-____.__ ._. _.

72.6 73.7 72.0 60.7 70.6 71.7

26.2 27.3 26.8 26.6 26.5 26.3 26.6 26.3 26.8 25.9 27.1 23.5 24.1 23.1 24 8 24.3 27.6 23.5 23.9 24.3 24.6 23.9

162.0 157.3 165.9 162.1 160.7 164.5 161.2 162.3 156.9 i626 163.7 171.5 170.6 188.2 170.0 168.8 1722 184.4 168.2 170.9 163.9 166.7

71.8

25.5

167.0

53.7 55.1 54.0 55.1 54.6 54.2 54.0 52.0 52.6 54.2 53.4 53.0

iXl.9 59.9 60.4 60.1 60.8 60.3 605 60.6 61.0 60.2 58.9 59.2 58.7 60.2

73.0 72.4 73.3 72.4 68.5 73.4 64.7 72.6 73.4 70.2 73.3 73.9 75.7 73.7

59.6 59.2 60.0 58.8 59.3 59.7

54.0

59.9

Cl@)

0.602

0.545

1.01

0.816

0.712

5.01

1.40

7.65

S,(e)

0.128

0.116

0.214

0.178

0.159

1.07

0.298

I.63

____________~_

__--_.._____-_

(Q Rejection is for a 3500 ppm Nail solution tested at 250 psi at a stirring speed of 1035 i-pm. (a) Flux is grams of permeate per hour per 13.3 cm2 of membrane area. cc) 2 is the mean value. defined as: 2 = Xxfrt_ (a) a is the standard deviation, defined as: u = {w - (&.#/n)]/(n -- l)jk. (c) Sm is the standard error of the mean, defined as: Sm = o/n’. l This membke has bben used for approximately 20 hours prior to commencing ti SC& of tests.

213 TABLE

I1

EFFECT OF COhCTWWR.ATiOS ON METAL REMOVAL

concentruCkm (ppm) .. -c __-+_~_v-..-.---.--*.,,..._~_ -.. ----- .._- _ .._ I 10 mu /twil __- __ _- ____ -_ _-.__--_r __. _.._..._----m---e_. ._*__---.- --__-1_ __-_.___.______ __--._.---__. v

!?!iLo

Al-3 cr*3 Fe-"

EL CTCla.Cl-

cl&‘= CU’Z &In-l __-__-~----___.~_

_~--_^-_~_

(a$ Under stmxktrd

99-u

___“-l___

_-...I

99.1 9s*(s

FE

99-i W*8

99.? 93.6 y6.9 96.5

99.4 37.6 96.3 30.8

99.2 91.4 97.1 91.1

_. *_c__ ----_-__t~--L-_-.

tczst ~~~~ti~n~ with an P&Clso ~rn~~~c

--+l ---a ,a*

and a 3XKl pp”

- ._.--_._n__-_

ru’aC1 feed

__-___

Solution.

Fe Cl, BaGI C-OCI,

most removal efficiencies were generally insensitive to f& concentration, The trivalent metals showed no variation with feed concentration and the divaknt met& were relatively stable, although they did exhibit a slight reduction in metal ient ~tj~n~ over the sntal ~~on~j~~~ for this formity in perfkmance. A rez~oua omogeneous feed is maintain the concentration polarization; region throu efficient surface turbulence promot icn. in the A total of ~~~h~~~ ~~ffe~~t heavy metai chior es is study- The leveI of Ejection of a given metal was foun

214

H. KIRK

0, 0

I

10

I

I

20

30

I

40

%

Fig. 4. Reverse osmosis their chloride salt).

so

&I

r

1

70

a0

JOHNSTON

r

90

3

Na cl -

membrane sehxtivity

for heavy metals (ali metals in the form of

,

in direct proportion to the NaCl rejection ability of the membrane. This increase was linear up to the level of maximum metal rejection but then remained constant. Example plots of this general behavior are shown in Fig. 3 for BaCI,, CoCI, and FeCI,. The slopes and in!ercepts of this type of curve were found to bc dependent not only on the metai but also on the counter ion employed. Consequently, the chloride counterion is employed throughout this study for the sake of consistency. For each salt examined, the linear least squares best fit straight line was drawn through the appropriate points. Results obtained in this manner are represented in Fig. 4 for the metal chlorides. The slope and intercept as well as the percentage metal removal at the specified NaCl membrane rejection levels of SO, 70 and 90% are summarized in Table IfI, for all the metal chlorides exalpined. The intercept value is the hypothetical metal rejection ability of a film in the limiting condition where it is no longer able to selectively reject NaCl from water. The reverse osmosis prqzess is generally conceded to differentiate between various constituents in a solution on the basis of their specific physicochemical

40.1 37.7 34.2 38.9 32.8 33.1 32.2 13.7 37.3 34.7 43.4 88.9 41.0 34.5

(2) IIn+ (3) Be+? (4) co+2 (5) Cd+2 (6) Coi2 (7) Cu'2 (8) Fei2 (9) Mgt2 (10)Mrv2 (II)Ni+2 (12)Pb+3 (13)Sn'? (14)Srt2 (15)Zn'?

-__-,_--_

88.3 83.8 80.8 85.1 81.9 84.1 82.0 73.5 H4.2 81.8 85.5 99.3 84.1 83.4

74.5 70.6 67.5 71.9 67.9 69.5 67.8 56.5 70.8 68.4 73.5 96.3 71.8 69.4

100 97,o 94.1 98.3 96.0 98.6 96.2 90.0 07.5 95.2 Y7.6 100 96.3 97.3

Y4.0

_-..-..__

ta) Cl- counterion tb) All data obtained from Rci,(45). w Basedon cu+nq = - 5,4cm"/moleat25°C.

Y7.3 100 99.1 100 100 100 _-_--___.___

88.1

82.1

NaClao NUCllO (A) (01 ..__._-,__.-_-

89.8 (16)AIt 71.1 93.6 (17)cl"+3 79.6 95.9 (18)Fei3 85.6 --.---_---------._..---_--

(I)

67.2

NaCln co,

-___.__-

_^___-__ .____ .-_

Percent nrctal rcttrv~~al

e....,

K"

_-.-.A_-_-F_---

httw"

.___~__-__-__-_

TABLE 111

._

315.1 5112.3 380.8 430.5 479.5 136.2 456.4 455.5 437.8 494.2 357.8 371.4 345.9 484.6

80.8

-..-_-I.-.:

-

- 58.4 - 55.7 --59.9 . . .

-29.0 - 32.4

-23.3 --22.0 .- 211.6 -- 30.8 -- 34.8 - 35.0 - 35.5 - 32.0 -28.5 -.- 34.8 - 26.3

..,. _.___.

.- 7.6 (-65.6) - 23.8 - 29.5 -37.2 -34.2 - 36.8 -3&R --30.6 -40,l - 5.5 -16.5 --20.0 -36.1

‘(0

.__-

3.6

,._

19.2

--_-..-

4.35 1103.3 -90.8 4.24 -89.4 1037 3.44 1035.5 -86.0 ^-_-.... _-.__ _.. ..- __ __.

5.HO 4.26 5.90 5.45 5.77 5.37 5.05 6.05 5.MO 5.85 5.17 4#03 5.85 5.55

0.689 0.658 0.665 0.65') 0.702 0.728 0,712 0.855 0.670 0.672 0.602 0.148 0.615 0.698 0.375 0.278 0.207 -- r

6.27

0.298

Shpc

26.Y8 52.00 55.85 . __

137.34 Y.Ol 40.08 112.40 58.93 63.55 55.85 24.31 54.94 58.71 207.19 118.69 87.62 65.37

39.10

__.

0.50 0.69 0.64 _-_-

I.35 0.31 0.9Y 0.97 0.72 0.96 0.76 0.65 0.80 0.72 1.20 I,12 1.13 0.74

1.33

! 5 5

1

2

5

z 3

!i 0

4 ii .. - .-_.. . _. ...- . .._.__ ,_ ___ _._. _ _________."__ ^___-. -_, - 8 Cution propwic’db~ (of ?S”C) PH of . ...._ -__ fj iopjlN -JlwJ p, 3,” Nd’hn M sall~riOn(krul/nn~le) (e,rr.) yi, g (cnr’/tttuk) (a.ttt.u.) (0) ._._ --.-. .__-_ _... _I,. . _ __ ___.__,____. _._

interactjons with the polymer fit s as well as their general sufution properties, C~~~u~~~~y~ it was of ~~~~r~st to try to ne some of the ~rit~~a which may in i&sweir as several of the specific parame bserved cellulose acetate membrane sekctivity for different heavy metals. As it is well reverse osmosis ~rn~~n~ exhibit very specific ~~te~~ti~~s with of chemical co ounds (3E, 32, 33) snty metals with the -me counter ion ar+zcompared against each other in an valence and having the umber of experimental variab& attempt tc minimize t fla alkaline eati cations were studied in sti havioral [email protected] of heavy metal salts shoutd be reasonably by the divalent metal chlorides, An insuf%ient number of . mctats in the monovalent and covalent states were studied to permit general toncfusions to be drawn- Table IfE pren%s a detaited c~mp~~~~o~ of oil metal salti examined, their rcjcctions by ccltufasc acetate membranes a,f different their relevant physicochemicat -solution ~n~e~ct~ons have not ivocafly cha~act~~~~ in terms of specific cht icaI reactions ur in terms ofthe t~~s~~~rej~tj~~ mechanism operating during the reverse osmosis process, Although revem osmosis is not an equilibrium prms, th~rrn~yn~~c crireria offer an attractive. method t0 a~~~o~~rn~t~ ce employed to define functional criteria for membrane dynamic concepts setectivity. Solution properties of the various salts also influence membrane ~~~~a~ce aad relevant ~aram~tcrs shall be presented where ~~~r~p~ate. Since many of the% solution prqerties a& ~~t~~~~~d~~t, it is di~~~~t to assess each on srnindividual basis. The approach taken herein is ta show their general correlation with &served membrane selectivity and to attempt to inter-relate dilfferent

(ii)

Tllermody fimic In an attempt

Cf ifp f

iQ

to asSeSs whether the therm~ynamic ~ute~ctiou~ which osrn~s~s separation of heavy metah werb d ion or water-ion interactions, the alkaline cart (I?&‘, Mg+2, CJa+l, Sr+‘, Ba+‘) were considered in detail. Three of tha [Ca+2, Ba+z) e~h~b~t~ co~s~s~~t pbys~~~ and c~e~~~1 pro~~~~, as wautd be SP, ed within a homologous series. Ma ium. generally behaves = a connal divaIernt cation although some of properties diverge slightly from the Group II, elements_ Beryflium, however, is not usually included with the other se of its te to ~~~e~~~ ~~~Q~i~~~y cQva~e~t members of this st electr;onegative counterions, no ion with borrdiag. Even this metal shall solut.ians of Be=+2 are known to exist as such (34). ansequentty, with the r~~r~~ta~ve e~~~e~~ of the ~l~a~~~~earths, y observed behavior is j~e~~~~ in Fig. 4 and Table fli-

REVERSE

OSMOSIS

REJECTION

OF HEAVY

METAL

217

CATIONS

Although the proposed thermodynamic criteria for membrane selectivity (Eq. 2) depends on the relative magnitudes of the ion-water and ion-membrane interactions, no quantified theory is yet able to predict a priori the extents of such competitive effects. However, by considering a limiting case in which one of these interactions is expected to predominate, the suitability of that situation may be assessed. Since the magnitudes of the partial molal free energies of hydration (APFrd) of the cations are conveniently available, this offers a logicA starting point. Differences in the partial mo!al free -nergies of hydration for different cations are expected to exert the controlling influenq: on membrane celect,‘vity if membrane-cation attractions are reasonably constant for the various cations. For the rejection of ionic species, coulombic effects would likely predominate such attractive forces for chemically similar entities. The absence of fixed charge sites on cellulose acetate reverse osmosis membranes does not in itself preventcoulombic forces from operating. A counter charge would he expected to exist on the oxygens arising from induced dipoles and weak permanent dipoles within the carbonyl groups and ether linkages (I, 13). This resulting charge would be significantly weaker than for ion exchange type membranes in which strong permanent dipoles exist. Fig. 5 shows the effect that the partial molal free energies of hydration appear to exert on alkaline earth cation rejection ability for the reference rejection levels of NaCI,,, Nat&, and NaCI,,. The observed relationship supports the concept that an ion with a small negative energy of hydration will be preferentially attracted to the membrane surface and consequently will be rejected to a greater extent. Such behavior sueests that the effects of the differences in the free energies of hydration significantly exceed the differences in the membtane-cation attractive forces, thereby making the ion rejection ability of the cellulose acetate membrane apparently dependent upon the solution properties of the various cations.

SO: SW

1

uo -A

Fig.

hydration.

5.

F,w

xa (lcml/mole)

Correlation of alkaline earth cation removals with partial molal free energies of

H. KIRKJOHNSTON

218

The linear dependence of cation removal efficiency on @Fd in Fig. 5 indicates essentially constant membrane-ion interaction energies for the various alkaline earths. Such constancy does not preclude the possibility that the absolute magnitudes of these energies may exceed the partial molal free energies of hydration_ Wheiher the membrane-ion interactions are occurring within the membrane or at the membrane surface, the relative.magnitudes of such interactions would be expected to remain reasonably constant for the various cations. The preferential transport of water during the reverse osmosis process is not obviously affected by the presence different cations and similarily the water transport does not appear to influence ion transport. Fig. 5 suggests that the extent of hydration of the ions in solution is not a controlling factor since the experimentally observed selectivity sequence corresponds to I,, which has been proposed to depend on unhydrated ionic radii. The effect of @F’” on divalent heavy metal rejection ability (Fig. 6) is not as pronounced as for the alkaIine earths although the same general behavior is apparent. When the cations being compared are from within the same group! such as the alkaline earth Group 11,. the chemical similarity enables the comparative effects of AFy to-be more sensitive indicators of relative rejection abilities Since most of the other divalent metals are from the same period but different groups. coulombic forces are necessarily different for the various cations and noncoulombic forces exert a variable influence that is dependent upon the chemical nature of the specific element. Consequently, the overall dependence of rejection ability on

of

m 2

k_ -‘AN ‘4

-A

5.

12-w_

A‘,, 4

0. 2

A

10

LA

8

---_

n

_--A_ --i

--3

00 =

W--b 12 --\*

4---

-A

Fig. 6_ Correlation

10

--__

F,““”

9

--

‘rz

--_-

~kcal/lllok)

of heavy metal removals

with partial molal free energies of hydration.

REVERSE OSMOSIS IEJECTION

OF HEAVY MEI-AL CATIONS

219

hydration energies is not as well defined for the heavy metals as it is for the alkaline earths.

In Fig. 6 and all subsequent figures the results obtained earth elements

are indicated

as blackened

for the alkaline points. The internal consistency within

the four alkaline earths (Mg, Ca, Sr, Ba) is sufficient to cause observed trends to be more pronounced than for chemiczlly dissimilar metals. Consequently the parameters examined in this study shall have two lines drawn through the experimental

data: one for only the alkaline earths (case I); and one for the all-inclusive

heavy metal cations (case LX). The general behavioral characteristics of ~11 the heavy metal cations are consistent with the trends apparent for the alkaline earths although the magnitudes of the charges are not as extreme. The observed rejections of SnC12 (Table III) are unusually high and do not correlate with the other divalent heavy metals. This is explicable by the fact that Sn+ 2 is unstable in aqueous solution (34) and is readily oxidized to higher valence states. On the basis of differences in interaction energies, the partial molal free energies of hydratio*n are able to be related most effectively and consistently to the removal of divalent heavy metal salts by the reverse osmosis process. Cation entropies in turn exert a controlling effect on ion-water interactions and thereby also strongly influence heavy metal rejections. Fig. 7 illustrates the experimentally obtained linear dependence of metal rejection ability on the entropies of the individual ions in solution (S;,,). The

Fig. 7. Correlation

of heavy metal removals with ionic eatropies at infinite dilution.

H. KIRK

JOHSSTOS

excellent correlation at the four reference rejection levels illustrated, shows a consistcat increase in cation rejection with increase in entropy. Since entropy is a measure of the degree of disorder or randomness in the system, it follows that rejectioc increases when it is likely that the cation-membrane interactions will be insufficient to orient the ions at the membrane interface (a condition of lower entropy). Such a hypothesis is consinent with the idea Jready proposed, namely that the ion-water interactions and equilibria efGctively control cation rem&al efficiency and ion-membrane interactions are sufficiently weak that they exert a minor in&en= on the removal of such solutes. An appropriate method ofquantitatively elucidating ion-solvent interactions ir, to examine

the partial

molal

volumes

of the various

electrolytes

at infinite

thermodynamic quantity is easily and consisotntly determinable by several different experimental techniques (35,36) and is a sensitive indicator of ion-ion, ion-solvent, and solvent-solvent efftas, but is particularfy appropriate to the study of ion-water interactions at infinite dilution (36). The partial molal volume of an ion is a function of temperature, pressure, and the concentration of

dilution (VT). This

other ions in solution.

Most heavy metal cations in aqueous solution are considered to be electrostrictive structure making ions (36) because of their tendency to orient surrounding water molecules around themselves to form multilayer hydration shells (37). This is a phenomena known as positive hydration (38). The partial molal voiume of an

Fig. 8. correlation

of heavy

ID& r&ovals with partial mojal volume at inlioitedilution

REVERSE OSXOSIS WECTION

OF HEAVY

221

METAL CATIOI;s

ion (VT) is the result of several different effects and can bc broken down into the genera) form (36): (3) where P;_,

is the crystal partial molal volume (the nonhydrated

ion in a crystal),

V&

is the electrostrictive

partial

molai volume,

volume of the vzicord is the

partial molal volume. and V’&,, is the structured partial molal volume. Such a partition of V; should not be considered absolute since the various terms necessarily overlap and mutually influence each other. Nonetheless, it is possible to evaluate each term with reasonable precision and they ccnsequently provide a

void-spati

semiquantitative

and

internally

consistent

indication

of

the forces

operating

in infinitely dilute solutions (36). The correlation of rejection ability with the partial molal volumes of the various heavy metals (Fig. 8) further sugg%s that ion-water interactions energetically control the selectivity of the reverse osmosis process for divalent heavy metal cations. between an ion and the surrounding

hydration

shell

(iii j Physical crifer io Although ionic entropy may be considered strictly as a thermodynamic property, it may also be regarded as a property which empirically incorporates the physical effects of the gram atomic weight (MJ, the charge
(4)

in which A is a defined Iength parameter and R is the universal gas constant. In this relationship the conventional single ion entropy of a species (Sr) in a hypothetical 1M aqueous solution is equivalent to ST,, since the value of S;+ rQ is zero by convention and:

(5) Although it is more appropriate to employ a thermodynamic criteria such as entropy to explain divalent cation rejection abilities, the partitioning of the entropy term in the manner of Eq. 4 encourages an examination of each of the contributing physical parameters. In the present study, all cations compared with each other were divalent, thereby reducing the dependence of entropy to the combined effects of atomic weight and ionic size. The correlations of each of these parameters with cation rejection ability are illustrated in Figs. 9 and 10 respectively. Unhydrated ionic radius appears to exert a more significant effect on cation selectivity (Fig. 10) than does atomic weight. Previously mentioned theories of membrane selectivity based on ion size are not totally unfounded. It is more appropriate, however, to

H. KIRK JOHNSTON 1T

I-

t

T-

0

Fig. 9. Correlation

Fig. 10. CorrAation

of heavy metal removals with gram atomic weights.

of heavy metal removals with unsoivated

ionic radii.

REVERSE

OSMOSIS

relate selectivities

REJECTION

OF HEAVY

to the various

METAL

223

CATIONS

ionic entropies

so that the combined

effects of ion

size, molecule weight and charge can be adequately incorporated. If the difference in ion-membrane interactions were energetically more important than the difference in ion-water interactions, the effect of ion six could be the controlling factor in cation rejections. The results depicted in Fig. 10 (Ba > Sr > Ca z=- Mg) are the exact inverse of what would be expected if this were the case. The increase in cation rejection with increase in unsolvated ionic radii is additional support for the concept that the difference in ion-water interactions is the energetically predominant term in Eq. 2. The fact that solute interactions with the cellulose acetate membrane are so uniform in this case suggests that different divalznt heavy metal cations are selectiveiy rejected durrng the reverse osmosis process on the basis of the energetic differences in their interactions

with the bulk water medium. CONCLUSIONS

Thermodynamic

solution

properties

appear

to exert the controlling

effects

on the selective rejection of heavy metal cations by cellulose acetate membranes. When cations are from the same group and experience approximately equivalent chemical interactions during the reverse osmosis process, relative solute rejection abilities are more sensitive to changes in these thermodynamic properties_ Interactions of divalent cations with a cellulose acetate membrane surface are generally uniform and are likely largely coulombic in origin. Selective solute rejection is dependent mainly on the interactions of the divalent cations with water in the bulk solution_ The overall controlling criteria for these interactions appear to be the partial molal free energies of hydration and the entropies of the ions in solution. ACKNOWLEDGEMENTS

The author thanks J. Reeves, R. Wardell, R. Dalrymple, and H. Huneault for assistance in the progress of these investigations. The analytical assistance by ttie Water Quatity Laboratory Network (Ontario Region) at the C_C.LW. was greatly appreciated. REFERENCES

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H. KiRK JOHNSTON

224 6. 7. 8.

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13. 14. 15. 16, ii’. 18. I9. 20. 21. 22. 23. 24. 25, 26. 27. 28. 29.

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