Calcium sulfate solubility in brackish water concentrates and applications to reverse osmosis processes; polyphosphate additives

Dcsulinurion, I5 ( 1974) 177-l 92 @ Elsc\icr Scientific Publishing Company,

Amsterdam

- Print&

in The

Ncth4nnds

CALCIUM SULFATE S0LUBlLl-fY IN BRACKLSH WATER CONCENTRATES AND APPLICATIONS TO REVERSE OSMOSIS PROCESSES; LEROY

POLYPHOSPHATE

B. YEAT-&.‘*

PAUL

K&c

ADDITIVES”

hi. LANTZ .\‘utiond

A\V

WILLIAXI

L. MARSHALL

ldvwutor~ , Qrth Kutqc, Tmn. 3X30

Cltcvnistr_v Divbion,

04

(Rczeixcd Fcbruar)

25. 19i-l: tn rc\isrvl form hlay IO, 1974)

( U.S. 4.)

SL:XlLI:\RY

The volubility of calcium sulfate dihydrate. which by precipitation can impede reverse osmosis (RO) processes, was determined at 25’C in three typical bracktsh water compositions and their concentrates. These waters corresponded to those used in U.S. Otlicc of Saline Water (OSW) RO plants at Gillette, Wyoming, and Webster. South Dakow, and that of a post-irrigation w.tter from the WelltonMohawk Canal in Arizona. Sodium hexametaphosphatc was shokvn, when added in small amounts. to produce an apparent increase in the saturation concentration of CaSO,. Latter, this concentration decreased to the solubility in the absence of the additive. Calculations of magnum water recoveries and other parameters of direct application to RO processes are presented. INlRODtiClION

The

extent

to

which

a saline

water

can

be concentrated

by

the

reverse

osmosis

(RO). or hyperfiitration, process (or by distillation) in recovering pure water is limited in part by the saturation precipitation of particular substances which may foul RO membranes. In a distillation process, a similar precipitation may form as scale on heat exchanger surfaces thereby decreasing the operating efficiency. Most naturally occurring brackish waters contain calcium, magnesium, and sulfate ions. and usually a small amount of hydroxide ion. Some substances of Eow solubility that may precipitate from these waters. for example Mg(OH),. CZI(OH)~, MgCO,. and CaCO,, can be eliminated by small additions of acid to a Research sponsored by the Membrane Processes Division, U.S. Office of Sa:me Water, and performed at the Oak Ridge National Labontoty, owrated by Union Carbtde Corporation for the U.S. Atomic Enerbt Commission. The experimen!al work was performed in the Reactor Chemistry Division of ORNL. h Analytical Chemistry Division, ORNL. c Health Physics Division, ORNL.

L. B. YEATTS.

17s the water. Calcium sulfate, its solubility m concentrates

ASD

P. M. LAN-U

W. L. MARSHALL

however. is not readily eliminated, and knowledge is necessary to establish the extent of concentration

of

that

may be obtained before saturation occurs. Of equal interest is the qualitatively observed effect of additives. for esample. sodium hexametaphosphate (SHMP), in retardin g the precipitation of calcium sulfate and other substances. Controlled studies have appeared to be needtd for understanding more about effects of particular additives. The ensuing results ond their interpretation could then be applied to preventing precipiration of calcium sulfate in both the \\aters under consideration and brackish waters in general where additives are used. It was the’ purpose of this study to determine the solubility at 25 C of (SyPsum) in three representative brackish waters and their conCaSOJ - ZH# centrates and also to determine the effect of added sodium hexametaphosphate solubility. The waters selected were those used in (SHMP) on CaSO, * 2H,O OSW RO plants at Gillette. Wyoming. and at Webster. South Dakota, and that from the Wellton-hiloha\\k post-irrigation canal in Arizona. Earlier, from many solubility studies of C&O, (and its two hydrates) in multicomponent salt systems. a generalized computer program was developed for calcuktting solubility limits of C&O4 at temperatures from 0 to 35O‘C (I). This present experimental study was made to test further the prediction of solubility limits of CaSO, and its hydrates in saline waters. with consideration of additives and with particular application

to RO processes.

EYPER’MENTAL

three

An eight

times

brackish

waters

(S x ) concentrated, mentioned

above

synthetic \vas

master

prepared

solution from

of each

“.malytical

of the

grade”

reagents and demineralized water. However. the molality of stoichiometric C&O, (multipIied by 8 x ) that would have been present in the natural brachish water was specifically excluded from the composition of these master solutions; an S x concentration of CnSO, would not have been soluble. Dilutions of each master solution were made volumetrically to produce stock concentrates as well as a 1 x stock solution. The compositions of the synthetic I x stock solutions are given in Table 1. In this table, however. the corresponding stoichiometric molalities of although actually not present Table I differed very little from those of the naturally occurring waters given elsewhere (2). The solutions were made slightly acid (corresponding to pH = 5.4 for the 1 x solution) with H2S0, to simulate the treatment of feed waters used in present commercial reverse CaSO, in the

in the natural waters have been included stock solutions. The compositions in

osmosis

plants. Sodium hexametaphosphate solubility was studied, was obtained

(SHMP), the additive from the Calgon Corp.

whose (Merck

effect upon Co., Inc.) as

CALCIUM

SULFATE

SOLUBILITY

IN BRACKISH

WATER

179

CONCENTRATES

_

__.. . ..-.-

.._-.----

---.

.

Gmpmtml

NCI K Cn-’ hlg hln

0.0038 o.ooo35 0.01078 0.0089 I -

O.OOSlS 0.00041 0.00353 0.00436 0.00002 -

:c Cl so ,a.‘* 502

0.00007 0.00030 0.0,116 0.000X7 0.00010

O.OWOl 0.000X 0.0106’, O.OOOS6 0.00058

HCO$ NOJ F PO4 Ionic strength

1 The stoichiomctric

-

-

0.00005 -

-

0.0840

0.0409

molality

of CaSOr

content

0.00016 0.03430 0.01’87 0.00047 0.00007 o.ooo13 0.0001 I 0.00003 0.0838

\\as absent from each stock solution

studies \\ith C~SO.I. ZHJO. 1)Total sulfate after acidltication with HzSO: p HCO?--

o.om77 0.00041 0.00611 0.00364 -

used

in the solubility

to pH =: 5.4.

after ac-idtfication.

the product. C&on. Calgon as impurities corresponding trace quantities (< 0.1 wt

is csscntialiy solid SHMP, but it contains carbonates to approximately 6 wt ?; CO?, and also contains 5;) of aluminum, iron, and silicon. A moderately

concentrated solution of Calgon was prepared l-2 hr before its use in the solubility experiments. In this paper, SHMP therefore refers to sodium hexametaphosphate in the form of Calgon. In general, excess solid CaSO, - 2H ?O (Mallinckrodt Corp., reagent grade). the stable saturating solid at 25°C. was equilibrated with volumes of the several stock solutions placed separately in 50 mi polyethylene bottles. These solutionsolid mixtures were rocked overnight (16-18 hr) or longer in a constant temperature water bath regulated at 25 + 0. I “C. After rocking was stopped, the solution phase of each mixture was sampled through a porous Pyrex (SiOz-borate glass) filter fused to Pyrex tubing. One volume of each sample was evaporated to dryness for determining the weight of water per ml of solution in order to convert molarity

180

L. B. YE4TTS,

P. .\I. LANTZ

AND

W. L. WARSHALL

to molahty. Another volume was used to obtain the total concentration ofcalcium. This vaiuc was established from a determination of the combined concentrations of calcium and magnesium by a semimicro potentiometric titration with standard disodium ethylenediaminc tetraacetate solution (EDTA; -0.01 M). The known concentration of magnesium in the brackish water \\as subtracted from this total concentration to obtain the concentration of caicil;m. SHMP, at the 5-160 ppm lekcis used, was found not to interfere with the determination of calcium. Additional details af the experimental procedure have been presented previously (I-3). The obtained an.dyticai molality of calcium in a stock solution saturated by CaSO, * 2H10 in that solution. It therefore - 2H,O represented the solubility of C&O, included the amount of stoichiometric CnSO, in the corresponding natural \\ater 11 I x that still remained dissolved in the concentrate. Soiubiiitles of CaSO, - ZH,O at 25’C in NaCl-H,O solutions varying from 0 to 6 m were determined for comparison with those values obtained and referenced earlier (4). The present values agreed within + I “,i with most of the previous results. and appeared to justify the experimental and analytical techniques used. Smce all three waters considered hate relatively high concentrations of SOiion (Table 1). some solubilities of CaSO, - 2H 2O in Na,SO,-H zO solutions rlt 25’ C were determined for comparison with results of cariier investigations (5.6). The molal solubilities obtained are plotted in Fig I against the moialities of added Na2S0,. Again. the present values agree well with earher results. and support both the equilibration procedure and analytical method. The decrease as the molality of Na,SO, increases to 0.13 ttz in soiubiiity of CaSO, - 2H2a is due to the increasing molality of the common ion, SO;-. 1shile the increase

03ri‘

r_._.__.--___-.__.__-___

_-. ._-.-_-._---.-L_.

ES-C

./co< :_-_ . -

/

j ! i

._

__

_

3 YEC;TS

G i

_..

LAr,TZ,

.

L(Il_L Arm WILLS

4

YEA:TS

i____._..___.___ 0

I

2

-

9

cr)()5

‘-

a-0

./OH

j

=’

_._._.__

_ / L-.-----i

.

MARSH&L!_

[J

A?40 MARSHALL _.__._.__1_

05

6YNEP [J

1 FRESEIuT

ChEV

see

PNr5

C+‘=M

_--.-._..--._-. (0

STUDY.

60, 73

__ I5

lO7i;

x547

l!93&]

et (!9631j -._._.

.__._.__J

20

25

~OZSO~(rnO.,lllf/J

Fig. 1. Sr3bility

of CaSOa - 2H~0 in aqueous Na!50~ solutions

at 25’C.

CALCIUM

SULFATE

in solubllity

SOLUBILITY

at higher

Na,SO,

IN BRACKISH

molaiities

WATER

CONCENTRATES

results

from

a dominant

181 ionic

strength

effect. The solubility set of experiments

- 2H,O in water alone was redetermined with each of CaSO, to assure that the solution-solid contact time was suffkiently

long for attainment of equilibrium. that the electrodes for titration were in good condition, and that the nnalytical reagents and endpoint indicator had not become cont;tminated or diluted. The average solubility in water at X’C from these results was 0.0152 + 0.0001 ;)I, which agreed well with the value of 0.01523 +_ 0.00016 nr reported

earlier

(.3)_

KESCLXG AND DISCt_!SSlOX

The composition of the Gillette well water gwen in Table I shows that calcium and magnesium are the cations present in geatest concentration and are nearly equal in moialit~, whtle sulfate is the dominant anion. Values for the solubllities of CaSO, - 2H,O at 25 ‘C in this water and in stock concentrates as high as 6 x in concentration factor (C.F.) are plotted in Fig. 2 and tabulated elscu here (-7). These solubilities represent the total molality of CaSO,lin the saturated solutions and, e\ccpt at one point. will differ from the amounts expected upon concentratin_c the natur,d vvater compositions of TabIe 1. as discussed later. For best describing the solubilities in the various concentrates. the values were comerted to solubility product quotients (Q,,,). iv hich equal the total calcium

___--_ GILLETTE ‘.\LTEP .-. .,._ _. .(___,

Fig. 3,. Solubility WY., Webster, S. Dak.,

’ _-i--

of CaSOa . ‘Hz0 at Z’C in stock and Wellton-Mohawk, Ariz.. brackish

solution waters.

concentrates

of Gillette,

L. B. YEA-ITS.

182 molality (solubility) the limiting quotient strength

(3,5).

pletely

ionized. For

(5.8). CaSOi

The

contrary

(DH)

was made some

descriptions

appeared

the several

Dcbye-Hiickcl

to

to

values theory

AND

W.

L.

>lARSI+ALL

total sulfate molality and where A;=,, represents as the solubility product constant) at zero ionic

assumption

the present

species

(1. 4. 7). With

times the (expressed

P. 31. LANTZ

be

that

dissolved

calcium

sulfate

is com-

experimental

evidence presented previously and calculations. the inclusion of a neutral unnecessary based on earlier. similar studies

of Q,.,,P for 3 siven set of concentrates. is applied lkhherc log Q/_, is plotted

an extended against I’.“,/

of n Siven solution (conccn1.51’ ‘). The term I is the formal ionic strength and is equal to l/2 Z,UI, ziz. where nr, is the molality of the ion i \vhich has the slope of the plot equals the ;I charg: of z,. At the limit of intimtc dilution. theoretical Debye-Hkkel slope. and the inclusion of the extended term. 1.5 I’:‘. :tllo\vs the empirtcal extension of the slope to h&h ionic strengths. The value, I .5, is a semiempi&;ll number found generally to provide the best fit at 25 -C of the chperimental solubilities. The ewellent adherence of solubility product quotients of CaSO, and its hydrates to the DH slope has been demonstrated previously bj several studies in l-l supporting electrolytes (I. 4. 7. 9. IO) at temperatures from 0 to 35O‘C. and formed the basis of the calculations (I) of solubility and of saturation concentration factors to be discussed later. (I + trate)

2

~~._~~~~~.~.~_~_._

---

-

--.

-

250t

I

IUEEYE-

“IN

PURE

WATER

hUCKEL

SLOPE1

.- -.

Fig. 3. Comparison of formal solubiliry product quotients for CaSOa - ‘Hz0 at XT function of /*!,/(I C I .5 I*#9 in concentrates of Gillette. WY., brackish well water with and without sodium hexamrtaphosphate (SHMP) present.

as a

C,iLClU\l

against

SULFATE

SOLUBILITY

IN RRACKISW

WATER

CONCENTRATES

183

are plotted Values of C&p (log scale) for Gillette water and its concentrates I ’ ’ (1 + 1.5 I’;‘) in Fig. 3. The dashed line represents the behavior of Q/,,

solutions (I). while the continuous curve is drawn through the concentrates in the absence of added SHMP. of Q,.sp f-,-om Gillcttc

in N&l-H,0 values The

fipre

hhows

that

(I/,,,

in a Gillette

concentrate

is slightly

greater

than

in a

N&I-H,O solution of the same ionic strength. This greater solubiIity in the Gillette concentrate 1s believed to result from the formation of MgSOO, ion pairs because of the preww: of ma~ncsium. thereby reducing the concentration of SO:-. Consequently. .tdditional CaSO, - 2H,O must dissolve to satisfy the \aluc

of &, (I, II). Fip. 3 shows also

the effect of initially added SHMP at a low level of 5 factor) upon the solubility of CaSO, - 2H,O in Gillette concentrates. The results arc erratic but do show that SHMP in these trace quantities produces ;I substantinl (initial) increase in the analytical molality of calcium. [The term* “solubility” (in quotation m&irks) and apparent solubility are used in this papor to dewnbe the molnlity of calcium in the presence of CaSO, - 2H20 when SHMP is present. It is difficult to consider that this molality represents a rrlre solubility, \\ hich is defined as .ln equilibrium (either stable or metastable) condition. J The polyethqlcne bottles containing excess solid CaSO, - 2H,O. Gillette \\ater concxtrates. and SHMP from the preceding experiment were left undisturbed at room temperature for I2 days. After this period they were “re-equilibrated-’ by rocking for 10 hr. The results when comprired with the solubilities presented in Fig. 3 agreed with those in the absence of SHMP. Apparently, the polyphosphate, \\hich is the “active” ingredient in SHMP. hydrolyzed to form the orthoppm/C.F.

(concentration

phospLtc upon standing :tt room temperature over rrn extended period of time. Although tertiary calcium phosphate [Ca,(PO,)J is very insoluble, the concentr,ition of phosphate ion from the hydrolysis of SHMP precipitated too little, if any, calcium to make the decrease in concentration of calcium detectable by our analytical method. The solubilitics of CnSO, - ZH,O shown in Fig. 2 represent values in concentrates of the three synthetic brackish waters but where, except at one point on each curve, either a deficiency (at high CF.) or excess (at low CF.) of calcium would bo present in the saturated concentrates if they were diluted for comparisons with the compositions in Table I. By obtaining the esact composition of the saturated \vater at the particular value of CF.. this deficiency (or excess) may be calculated. Also, an experimentally derived value of a saturation C.F. is obtained that applies exactly to the particular watercomposition. To make these calculations, we must remember that the compositions of the stock solutions at 1 x may be obtained by subtracting a stoichiometric amount of CaSO, from each of the three water compositions given in Table I. (For Gillette water, delete 0.01078 moles

L. B. YEATTS.

184

P. M. LANTZ

AND

W.

L. SXARStfALL

Ca and thus subtract 0.01075 moles from the total sulfate to obtain 0.01038 r&es SO&.) The folio\\ing set of equations were used for the calculations: .4 - R -t- C = D/E

CfF=

100.(I-GIN)

=

D

(1)

f

(2)

G

(3) (41

= J

\\ fwre. at I x. which cyuals 0.01038 .4 = molatiry of sulfate m stock solution ~1 for Gillette water. R = C.F. of stock concentrate, a3 plotted in Fig. 2. molal solubility of CaSO, . 2H ,O at R. C= /I = total moiality of sulfate at B. E = totaf molality of sulfate in the natural \vater at I x . \\hich equals 0.021 16 I)I for Gillette water. natural water depleted or enriched by F SC saturdtron C.F. for particular calcium. where two i -+- cations replace each calcium ion. G= actual motality of calcium at 1 x diiutcd from vafue at F. or B in rclat~on to the stock concentrate. H = actual molality of calcium in the natural \xater at 1 x _ uhich equals 0.01078 ~82for Gillette water.

Fig_ 4. Effect of cdcium depletion upon the eqxrimcorafly derived saturation factors (F) for Gillette, Wy., Websster, S. Dak.. and W&on-Mohawk. Ark. brackish \#aters at 25°C:

saturating solid is caS0~ - 2H~0.

CALCIUM

J pitrtXUlar

SULFATE

SOLURILITY

IN BRACKISH

= per cent depletion. natural wnter.

WATER

or enrichment

CONCENTRATES

(negative

sign),

185

of calcium

in the

The values of saturation C.F. (f) calculated for Gillette \\nrer, and for the other t\vo brachish waters. are plotted in Fig. + against the calculated cent

depletion

of calcium

(J).

[Do

not

confuse

saturation

C.F.

(F)

of

Figs.

also per 4-6

plants. ion exchange with stock solution C.F. (B) in Fi_c 2. ] In RO dcalination replacement of calcium by sodium is the method mos;t likely to be used in depleting calcium from a brackish water: therefore. equations I-4 are valid for this process of depletion. We observe in Fi_c 4 the large incrcnse in saturation C.F. upon remo\lng calcium from the brackish \\cltcrz. The vrrlues for solutions enriched in

-.

Ili*bd
S. hk..

UOlei

I~‘cllron-;\foii~~~~ k Cunul, 0 ‘0 30 40 50 60 70

3 32 3.90 4.30 1.83 5.56 6.59 8.25

3.30 4.10 4.70 5.46 6.44 7.G) -

Ari:., 2.79 3.32 3.7s 4.16 4-81 5.74 7.12

)t nrcr 1.7s 3.37 3.95 4.64 5.4s 6.46 7.60

IS 30 36 50 57 66 75 Y6

36 51 59 65 75 -

70 74 77 79 82 Y5 l38

70 76 79 82 84 87 -

64 70 73 76 79 83 86

64 70 75 78 8’ S-l 87

18 19

;- Values based on a concentration polarization factor (C.P.F.) of I (see Eq. 6). b Calculatrd and experimental values here are the results obtained upon substituting cd and cyxrimcntal C.F. valurz, respectively, mto Eq. 5 in te\t.

the calculat-

1.. B. \‘EAllS,

I86 calcium

P. M. LANTZ

AND

\\‘.

I.. \I..\RStIXLL

\\ere not included. but \\ere used for eAtending the curtes to zero depletion. The computer program published earlier (I) was used to calculate indepen-

denti? saturation C.F.‘s at various levels of calcium depletion in e\pximcntally derived results in waters. A comparison of the water with the values calculated by the generalized program T;ible II. The calculations b> this earlier program predict very solution un5,lturation for Gillette water.

i. ._..- __.

_

_

.

_

.

--

the three brackish Fig. 1 for Gillette (I) is included \vcll the limits

in of

-- --

E\perimentall> dcrlwd \aturation concentration fxtors were presented previously (2) that differed somewhat from the vJues in Table II. Ho\\ever, thebe earlier ~~1~s \vere approximate bectusc ewct c:kulations of the compositions of the concentrates at saturation wxc not made. The present experimental volucs tTrible II) of saturation C.F.‘s ;1re considered to be more exact. and they show closer agreement with the calculated results. The r\perimen!al of calcium

results

In Fig. 5 cslculated

show that tmdepleted

Gillette

in Fig. 4 dnd a curve

of suturritlon

C F. $5 depletion

b> the computer program dc\elopcd earlier (I) w,tter can bc concentrated very little before prccipi-

Lition of CaSO, - 3H20 begins. Approximately 50”: of the initial calcium must be removed from this \\anter to enable the saturation C.F. to be doubled. [Changes to account for a replacement of calcium by sodium were made in the ionic strengths before saturation C.F.‘s were calculated (I).] Included also in Fis. 5 is a calculated curve of C.F. IX equal depletions of both calcium and magnesium. This cur\e also W:IS calculated by the computer program given else\\here (I). These comparative plots bhow that Gillette \\ater can be concentrated to a slightly greater extent if only calcium is depleted rather than both calcium and magnesium. The formation of MgSO: ion-pairs when magnesium is present effectively lowers the concentration of SO:ions and permits a sliphtly higher concentration of Ca’+ ions to

CALCIL’M

rcmi.iin and

SULFA-I-E

SOLUDILITY

in solution

(I, II).

e41.131 removal

without

IN BRt\CKISlI

WATER

I’,,) =

V~tluc~ of %lWR tion

C.F.

lOO-

bnscd

for Gillette

IOO’(Sat’n.

on both

water

187

r\t 60”,, depletion (Fig. 5). the satur.itlon of m~gntsium differ only by --G]‘,.

An important parameter is the per cent \shlch may be calculated by the equation. MWR(

CONCENTR.\TES

\\atcr

recowry

with

(MWR),

C.F.)

(5)

c\pcrimcntnl

at \ariouh

maximum

C.F.‘s

dcplctions

~tnd c;tlculittcd of alaurn

(I) LiIlucs arc included

of

saturnin Tabic

II. Thus. only kforc C3S0, concentr:ition inltl:tl ci~lciunl

IS’!:, of the Initial fcrd water In the KO procas could be rccovcred - ZH,O precipitates if no calcium is remwed initially anci if the polarization fxtor. dcfincd belo\\. equals 1. Hox\evcr. at 60”,, depletion of calcium. the calcul:ttcd MWR is 57”,,: the vuluc is 55’1; when and

mapcslum

are both

depleted

by 60”,,.

It a;1s desirable alzo to c.kulatc the cffcct of concentration pokization upon aturarion C F.-s when calcium is dcplctcd. The concentration polarization factor (C.P.F.) IS defined ~1s the rxtio of the concentration of a given ion at the wall or membrane surfax to that In the bulk solution. Althouph the C.P.F. is i\no\rn to differ for each ion. the .lppro\lmation W.IS made that it is the same for all ions in the particular \\ater in order to simplify the calculations. Calculated curves of saturation C.F. IS depletion of c.llcium for Glllcttc \vater are plotted in Fig. 6 for several v~~lues of C.P.F. \\ here

Sst‘n.

,,; ______

C.F.

= Sat’n.

C.F.(C.f’.F.

=

_._ .-_. ___ ________._______

l)‘(C

P.F.).

.._....-. -...

(6)

..__ _.-l !

Fig. 6. Effect of rcmobing cnlc~um from Gillette. Wy.. brackish ~alculktcd saturation concentration factor (F) at korious conccn!r.ttmn (C.P.F.) at 3 C; saturating soliJ is CaSOa . ZHzO.

~cll water upon the polarirrttion factor5

L. R. YEATTS.

18X The preceding makes

Fig. 5 shotis that the initial

only n small differcncc

calcium

1~ rcmoved:

removal

in the saturation

thercforc.

P. XI. LAST%

AND W. L. MARSIIXLL

of both caicium

C.F.

the effect of removing

values from

and magnesium those when only

both cations was not consider-

cd hex.

The included

synthetic in Table

composition I. The

of the

dominant

Webster. South in decreasing

cations

Dakota. well water order of concentration

is

with only trace amounts of mngneslum. calcium. and potassium. manganese and iron present. \\hile wlfr\te is the dominant anion. The solubilities - 2H ?O at 25 ‘C in this x\;Itr’r .Ind its stock concentrates to 8 x were of CnSO, determined by both (a) rocking overnight for 16 hr and (b) stirring the solutionsolid mixtures viporousiy for I hr. The soiubilities obtained were so nearly identical b> the t\\o methods :h:it equilibrium in the system is practically assured. These

are

sodium.

solubilities

ale plotted

of Qfl,, (log scale) from the data

m FIN. 2. Values

are plotted

in FIN. 7 against

Hz0

I ‘;‘/! I -+ 1.5 I I”). qain :\ith the solubility behavior in NaCI:ts :hc reference. The greater solubilitics in Webster water than in solutions at rhe same ionic strength again reflect the presence of mag-

serving

NaCI-Hz0

nesium

ions as explained

Shown determned

also values

above. the effects

in Fig.

7

of Q/,p

for CrrSO,

arc

__ .-.-_-_--I

SHUP

‘EtSEUT

.

SHMJ

PRESEsUT

(5 CD~

This

SHMP

x\hich

increase

is quite

Incrasc

the

regular

and

..-----._- ._ 25

0

of .tdded

- ZH,O.

‘C

per C = 1

s

6-

I-

1VVfATEt?+5psm

1. .-...-. -

2

/

!

,*-’

./

/ / i / L_._-_

31

/-

/

‘IF1

FURE

SHMP

I

WATER

,

i

/ ---_

02

.-

..__.

09 ‘1 I 2;(!*,

--_.___A

04

51”2/2,

Fig 7. Comparison of formal solubility product quotients for CaSOp as a function of I’,‘/(I + 1.5 /Id’) in concentrates of Webster. S. Dak.. brackish and without sodium hevametaphosphate (SHMP) present.

- 1HzO at ZS’C well water with

CALCIUV

SULFATE

reproducible

SOLUBILITY

IN BRAClW31

,IS the concentration

to 40 ppm in ,Y x

water

of

(S ppm’C.F.).

WATCR

COSCENTRATES

is increased

SHMP

from

189 5 ppm in I x water

However.

Fig. 7 shows that a 4 x increase in SHIMP to 20 ppm/C_F. does not produce a large incrcasc in the modality of calcium (or a much larger increase in Q,-sp); instead, there is an obvious, erratic effect upon stability. These results in the presence of 20 ppm SHMP/C.F. were obtained by equilibrating the phases overnight (16 hr) \\ith gentle rocking. A run with stirring for 1 hr to equilibrate the phases did not yield a more regular pattern for the “solubihties”. The experimentally derived values for the saturation C.F. are plotted In Fig. 4. and values at mtcgrdl per cent depletions of calcium arc compared wth calculated \aiucs in Table II. Atthough there is a moderate difference bctwzcn the t\\o sets of values for the saturation C.F., the two sets for maximum water rccobcry (Table II) ‘ire in good agreement. These results show that the initial well water can be concentrated by a factor of 3.3. or 70:‘; of rhc water may be recovered bcforc CaSO, * 2HL0 can bc expected to precipitate on the basis of a C.P.F. of I. Or if rhc initial calcium content of the \\cll water is depleted by 50”;. .lbout 84:‘/, of the water may be reco~erablt. Calculations with the earlier computer program (I) show that the saturation CF. is rcduccd slightly for Webster water by removing equal amounts of both ci~lcium and magnesium

instead of cuhum

:&xx.

At SO”, caicum

depletion,

a

maxtmum

\\‘ater recorery of 82:;, is calculated versus Sl “; rccovcry when both and magnesium arc depleted by 50’:;. The effect of diffcrcnt concentration polarization factors (C.P.F.) upon the saturation C.F. and maximum water recovery of this brackish \\ater as the

c.llcium

concentration

of calcium

is reduced

may

be calculated

by Eq.

5 and

6. As the

C.P.F.

increases from I.0 to 1.5 when no calcium is removed, the saturation C.F. decreases from 3.30 to 2.20. and the MWR decreases from 7031 to 55 ‘,?A.At 5O’Ji calcium depletion and as the C.P.F. rises from 1.0 to 1.5, the experimentally derived MWR decreases from 84:~; to 77:‘:. Wdtorr-

Mohawk posl-irrigatiott corral ut Arizotta

The Wellton-Mohawk nates

from

irrigation

waters

Canal in Arizona which

have

contans

percolated

brackish through

\\ater that origithe

soil

to

wells.

These well waters have then been pumped for disposal in the Canai. The synthetic composition which closely approximates the Wellton-Mohawk post-irrigation water and which was used for the solubility studies is included in Table I. Of the cations. sodium dominates, with much smaller quantities of calcium and magnesium. Chloride is the dominant anion. while the molarity of SO:- is approximately l/3 that of Cl-. The experimental solubllities of CaSO, - 2H20 are plotted against the stock C. F.‘s in Fig. 2, and in Fig. 8 values of QrSp obtamcd in this water and its concentrates are plotted against I ‘/rj(l + I .5 1”‘). Fig. S shows that these so1uvalues of QIsp are nearly the same as those obtained earlier in NaCI-H,O

I.

190

/

(ieJ

L____...-_

._..

O!

_.--_-_.--.

03 .. ‘6_ >(‘..

B. YEATTS.

_....

P. XI. LAN-I-Z

AND

W.

L. hl.\RSHALL

I 2

04

c3 5.- ‘6, L,

FiS. 8. Cornparlson of the formal solubillt) product quottents for CaSO: * ?HJO at 3 “C as a functlon of I* 2 (1 - 1.5 11’2)in concentraks of Wellton-!Uoha\\ h. Ariz., pocr-irrigation \wtcr wirh and althour sodsum hcvametaphospharc (SHMP) prw?nt.

in the post-irrigation water apain are attributed to the presence of magnesium. The effect of added SHMP in apparently increasing the QIrp‘s at the different concentration levels IS shoun also in Fig. 8. Houever, the solubility of CaSO, 2Hz0 in Wellton-Mohawk water concentrated above 2 x does not seem to be Cons.

atiected

The

any

slightly

higher

values

for

Q,sp

more

by SHMP at 20 ppm,C.F. than at the 5 ppm/C.F. level. In the increases the concentrates below 2 x and in pure water. SHMP at 20 ppm/C.F. molaiity of calcium greater than SHMP at 5 ppm/C_F.. but not by the factor of 4 N hich the SH MP concentration under_goes. Fig. 4 includes the experimentally derived limits of concentration of Weiltonhlohawk water after depletion of calcium from 0 to SO”‘,. The original icater therefore can be concentrated nearly 3 x at 25 ‘C before precipitation of CaSO, 2H,O is expected. At this 3 x concentration. 64 ‘?: of the water can be recovered from the original We!lton-Mohawk post-irrigation water before CaSO, - 2H,O precipitates (Table II), again for a C.P.F. of 1. This recovery can be increased to There is good agreement between Y2”< by removing 50 od of the initial calcium. experimental and calculated saturation C.F.‘s up to 30”/, initial removal of the calcium (Table Ii): a moderate divergence occurs at greater removals. Again, calculations (i) show that the removal of equal amounts of both calcium and

CALCIUM

SULFATE

SOLUBILI

I-Y IN BRACKISH

\VATER

COSCEN

CRAI ES

191

magnesium does not change significantly the values f-or saturation C.F. or maximum \\ater recovery \\ hen compared with the removal of calcium alone. The effect of- calcium

depletion

upon

mawmum

water

recovery

for,,Wellton-

Moha\\ factors

h water also may be calculated for various concentration polarization by Eqs. 5 and 6. When no calcium is removed. the experimentally derived from I.0 to MWK decreases from 6-t:,, (Table II) to 46‘,‘,, as the C.P.F. increases 1.5. At 50”;, calcium depletion. the corresponding v.dues of MWR arc 82y, and 73 “,;. respectively.

The determined solubilities of CaSO, * ZH ?O at 25 ‘C m concentrates of rcprebcntnti\e brachish tknters she\\ that formal solubllity product quotients in NaCl-H -0 solutlonslat the~same~ionic (Q/J arc t#Ircatcr th.tn those obtained to the presence of m,lgnesium strengths. These increwx in Q,-.,, are attributed \\hich may form ikf$SO~ ion pairs. requiring further diswlwion of CaSO, - 2H,O three

to satisfy the actual solubilrty product quotient (I, II). The good ugrecmcnt undcl various conditions between saturation concentration fxtor obtamcd from the experimental

the vnlucs for the results and those

calculated u\lng a computer program (I) indicate that the calculation4 method is A condcnscd program f-or applicaadcquatc for use in rvxluatin, (x RO processes. tion to RO desalination is given elwvhere (2). The concentration polarization factor would appc‘tr to be most Important in determinIng the extent of concentration of a brackish water brforc fouling occurs. For pracwxl applications. thercfore, calculated or experimental values of satur.ltion C.F. might need to be reviwd on the basis of the expected C.P.F. (see Eq. 6). The addition of SHMP at 5 ppm and 20 ppm levels to each of the three brackish ivaters shops that the solubility creased by SHMP. However. the increased exact, or even regular, function of the

of C&O, - 2H10 apparently is In“solubrlities” do not appear to be an SHMP concentration. Increasing the

by a factor of 4 times (3 x ) does not result in a comparative hloreoker. most of the experimental results were erratic in bchabior. We propose, therefore. that SHMP may peptize some of the pov.Jcred - 2H ?O to form a stable colloid (I?), thereby apparently raising particles of CaSO, Later, hydrolysis of SHMP may eliminate peptithe solubility of CitSO, * 2H,O. zation and allow the concentration of CaSOJ to approach the (true) solubility of cuso, - 2Hz0 in the absence of added SHMP. Some adsorption of SHM P by the macroscopic solid particles of CaSO, - 2H,O may also be responsible for reducing the effectiveness of SHMP. In reverse osmosis processes, where solid - 2H10 is not yet present, SHMP muy bc considered to act as an inhibitor case, to occur and providing of crystal nucleation, thereby zllowin, ‘r supersaturation transient concentrations perhaps in excess of those given in this paper.

concentration increase

of SHMP

in “solubility”.

192

L. 8. YEA-ITS, P. hl. LAh’TZ AND W. L. XIARSHALL

REFERENCES 1. w. L. hl UiSH.\LLand R. SLUSHER.f. C/rcl?J. &IX. DfJfU, 13 (1968) 83. I!. L. B. YE&n%, P. X1. LA\%? and \V. L. MAILSHALL,Solubiiitics of Coicium Sulfite Dih_wirutc ut -75. C in Brackish Warcrs mrl 77rrir Cmzccntrutrs: Effect of C&on Additi~r and Prdiction.c for Re\rr.w Ostmuis Proccmes. Oak Ridge National Laboratory Report. ORNL-4914 (1973). 3. L. B. Yr-\m and \V. L. ~~UUHALL, J. Ciwn. Ehg. Darn. 17 (1971) 163. 4. W. L. M~RSIIALL and R. SLUSHEH.1. Php. C/lent.. 70 (1966) 4015. 5. L. B. Yux-rs and W. L. MARSHALL. J. Piry~. Chm.. 73 (1969) 81. 6. A. E. H~_L and J. H. WILLS. J. .-lmcr. Gem. Sot.. 60 (1938) 1647. 7. W. L_ M ~SHALI.. R. SLUSHERand E. V. JOXFS,J. C/rent. Ehc. Duta. 9 ( 1964) 187. 8. R. KALYAYAR~WAS. L. B. YEAITS and W. L. M-\RSHALL, J. C/tent. Tbcrm~I.. 5 (1973) 809. 9. W. L. M,XRSHALLand R. SLUSHER.f. Ciwn~ Titermod, 5 (1973) iY9. 10. R. ~~LY-\~~AR~~AN, L. B. Ymm and W. L. hkUISHXLL. J. Chenr. Thernd.. 5 (1973) 891. 1I. W. L. %xRSHALL, J. Pir>s. Citent.. 71 (1967) 3584. 12. D. H. SOLOWOX and P. F. ROLFE. Desafinufion, 1 (1966) 260.