WATER RESBARCH. VOL. 16. NO. 5. 1982

Continued from front cover]

GENKI MATSUMOTO 551 Comparativestudy on organicconstituents in polluted and unpolluted inland aquatic environments-111. Phenols and aromatic acids in polluted and unpol- luted waters

MEHMET BERKUN 559 Effects of inorganic metal toxicity on BOD-I. Methods for the estimation of BOD parameters-I1

Mercury in the River Mersey, its estuary and tribu- taries during 1973 and 1974

D. AIREY and P. D. JONES 565

LESLIE BROWN, MICHAEL M. RHEAD, DAVID HILL and KEITH C. C. BANCROFT

579 Qualitative and quantitative studies on the in sifu adsorption, degradation and toxicity of acrylamide by the spiking of the waters of two sewage works and a river

D. FONTVIEILLE and M. RENAUD

WHEI-CHOU JEN and R. G. BELL

593 A method for estimating the respiration of mud com- munities in shallow running water

Influence of temperature and time of incubation on the estimation of bacterial numbers in tropical surface waters

605 Transport of iron, manganese, cadmium, copper and zinc by Magela Creek. Northern Territory, Australia

601

BARRY T. HART, SIMON H. R. DAVID and PAUL A, THOMAS

HALLVARD ~ ~ P ~ ~ A A R D and SUPORN KCJOITATEP by reverse osmosis

S. Y. IP, J. S. BRIDGER, C. T. CHIN, W. R. B. MARTIN and W. G. C. RAPER

613 Removal of humic substances from natural waters ,

621 Algal growth in primary settled sewage. The effects of five key variables

R. J. BETEMEYER, - 633 Product inhibition in the acid forming stage of the A. J. C. M. MATITIIJSEN, A. COHEN and C. BOELHOUWER

A. H. WEBB

anaerobic digestion process

641 Weak acid concentrations and river chemistry in the Tovdal river, southern Norway

649 The role of suspended solids in the estuarine geo- chemistry of mercury

J. E. RAE and S. R. ASTON

655 Eil'ec~ of mnpncdum cind cnlcirim precipitntion on coauiilntion-tlocciiliit ion with linia

R. CARE, J. D. MORRISON and J. F. SMITH

D. ROY, E. S. K. CHIAN and R. S. ENGELBRECHT ozone

663 On the limits of detection of traces of volatile organics in water, using amberlite XAD-2 resin

667 Mathematical model for enterovirus inactivation by

R. J. LARSON and S. L. SCHAEFPER

675 A rapid method for determining the toxicity of chem- icals to activated sludge

JOHN P. HASSEIT and Effects of dissolved organic matter on adsorption of MARC A. ANDERSON hydrophobic organic compounds by river- and

sewage-borne particles

687 A sampling study of the brown seaweed, Ascophyllum nodosum as a marine monitor for trace metals

681

M. E. WOOLSTON, W. E. BRECK and G. W. VANLOON

Continued on inside back cover

' Printed in Great Britain by A. Wbeaton & Co. Ltd. Exeter 309

Wocer Res. Vol. 16. pp. 729 to 737, 1982 Printed in Great Britain. All rights reserved

0043- 1354/82/050729-09SO3.OO/O Copyright 8 1982 Pergamon Press Ltd

REMOVAL OF HEAVY METALS FROM ELECTROPLATING RINSEWATERS BY PRECIPITATION, FLOCCULATION

AND ULTRAFILTRATION

ERIK R. CHRISTENSEN and JOHN T. DELWICHE Department of Civil Engineering, University of Wisconsin-Milwaukee,

Milwaukee, WI 53201, U S A .

(Received July 1981)

Abstract-CJhrdEm, nickel, copper and zinc can be effectively removed from electroplating rinsewaters by hydroxide precipitation, flocculation and ultrafiltration. Prior to precipitation, chromium is reduced from the hexavalent to the trivalent form by ferrous sulfate and cyanide in copper and zinc rinsewaters are oxidized by sodium hypochlorite. Minimum metal concentrations in the permeate from separate batches of chromium, nickel, copper and zinc rinsewaters were found to be, respectively, 0.17 mg I - ' Cr (T), 0.26mgI-' Ni, 0.30mgI-' Cu and 1.84mgI-I Zn. These solubilities are in good agreement with the theoretical solubility curves, except for copper where the formation of stable copper cyanide com- plexes appears to increase the solubilities at least two orders of magnitude relative to those predicted on the basis of the equilibrium constants for copper hydroxides and oxides. A simple mass balance model, assuming concentrate recycle and constant metal concentration in the permeate, is adequate for the prediction of feed and permeate concentrations as a function of the volume filtered up to a relative volume of about 0.3. Above this value, the feed concentrations are lower than predicted, apparently because of entrapment of metal precipitate in the strainer. Water recoveries are strongly dependent on the specific metal removed and are found to be 24% for Ni with a 0.20pm membrane, 10% for Cr with a 0.80 pm membrane, 6.5% for Cu and 3.7% for Zn, both with a 0.45 pm membrane.

INTRODUCIION recovery and water reuse can also be achieved by evaporation fBhatia & Jump, 1977), a process that

brane separation (Gregor & Gregor, 1978).

lation and ultrafiltration is an interesting alternative

of metal precipitate in concentrated acid, could lead

Removal of heavy metals from electroplating rinse- would appear relatively costly compared to mem- waters prior to the discharge of these rinsewaters into

the enactment in 1972 of the Water Pollution Control

this legislation is that most of the chemicals in elec-

Public sewerage systems has become a necessity with

for

Removal of heavy metals by precipitation, floccu-

treatment process which, if followed by redissolution

to essentially complete metal recovery. This method suggested but not investigated by Porter et a\,

Law 92-500)* The

troplating rinsewater, such as cadmium, chromium. copper* nickel* zinc and cyanides* are highly toxic and advcrscly can aNect the biological treatment in pub- licly owned treatment works. In addition, if their levels nre not significantly reduced at the discliargc

high concentrations, even after passage of the waste- through the publicly Owned treatment

with following detrimental consequences for the biota of the receiving waters.

wastewaters include cyanide oxidation of cadmium, copper and zinc rinsewaters, hydroxide precipitation, flocculation, clarification and possibly sand filtration. Chromium must be reduced from the hexavalent to the trivalent state prior to hydroxide precipitation (US EPA, 19794. Sulfide precipitation can be used for most metals to obtain even lower residual metal levels than what can be obtained by hydroxide precipi- tation.

If recovery of metals, or an exceptionally clean ef- fluent is required, ion exchange, electrodialysis, or reverse osmosis can be applied. To be efficient, these processes must$ however* be preceded by Pretreatment such as precipitation and carbon adsorption. Metal

(1971). Improved membrane techno~ogy in the past decade has enhanced its potential, The process requires simple chemical pretreatment prior to ultra-

ures (io-ioopsig) than osmosis and is con- siderably less critical to regulate than electrodialysis.

The advantage of ultrafiltration Over conventional clarification and sand filtration is the expected more

size, e,g. o.l ~ m , of ultrafiltration The time constants involved in ultrafiltration are also much smaller because the method is not dependent upon gravity settling of particulates.

The purpose of the present study is to evaluate the feasibility of removing heavy metals (Cr, Ni, c U , z n ) from electroplating rinsewaters by precipitation, flocculation and ultrafiltration.

point, these chemicals could remain in undesirably filtration. Ultrafiltration operates with lower press-

Conventional treatment methods for electroplating complete solids separation because of the small pore

MATERIALS AND METHODS

Membranes Initially, polysulfone membranes of 0.002 ym pore size

were used. However, membrane fouling resulting in low

730 ' ERIK R. CHRISTENSEN and JOHN T. DELWICHE

permeate flux and ineffective solids separation severely limited the usefulness of these membranes when filtering chromium rinsewaters treated with ferrous sulfate and sodium hydroxide. Membranes of larger pore size were therefore used in all subsequent runs. Based on additional factors such as strength and pH tolerance, membranes of 0.20-0.80pm pore size made from an acrylic copolymer were chosen. This type of membrane has a nylon support for additional strength and durability.

Elrctroplutitiy rinsewurers Four types of plating wastes were obtained from a major

Milwaukee based electroplater. For each ultrafiltration run, a 47-95 I. sample of Cr, Ni, Cu and Zn rinsewater was picked up and run as received and with chemical prelreat- ment including at least two different pH levels.

The composition of plating baths was as indicated by Mazia (1980). Major constituents in the chromium bath are chromium trioxide CrO, (240 g I - ' ) which reacts with the water to form chromic acid, and sulfate which acts as a catalyst. A low level (8 gl- ' ) of trivalent chromium is necessary for bath stability. The average Cr(T) concen- trations in the three coutercurrent rinse tanks was 78 mg I-'. The nickel rinsewater came from the same pro- duction line as the chromium rinsewater. The plating was arranged in a two-step operation so that nickel plating preceded chromium plating. The main constituents of the nickel plating bath are nickel sulfate (300gI - ' ) nickel chloride (45 g I-'), and boric acid (37 g I-'). The plating solution is commonly referred to as the Watts bath. Nickel sulfate serves as the principal source of nickel ions, nickel chloride improves anode corrosion and increases conducti- vity, whereas the boric acid provides whiter and more duc- tile deposits. The nickel content of the rinsewaters varied from 40 to 370 mg I - ' .

Both of the copper and zinc plating baths contain cyan- ide which acts as a chelating agent to keep the metals in solution. The main constituents of the copper plating bath are copper cyanide CuCN (58 g I - I ) and sodium cyanide (105 g I-'). The concentration of copper in the rinsewater averaged 31 mgl-'. The zinc plating bath contained zinc cyanide Zn(CN)Z ( 3 4 ~ 1 - I ) , sodium cyanide (19 g I - ' ) , and sodium hydroxide (75 g I- ' ) . The average zinc content in the rinsewater samples was 28 mg 1 - l .

Cltutriicul prcircuitmwi Chromium was brought to the particulate phase by

chemical reduction and precipitation. Ferrous sulfiite, FeS04.7H20, was added in excess (2.23 g l - I ) at a pH of between 2 and 3 to complete the reduction of Cr from the hexavalent to the trivalent state. Following reduction, the pH was readjusted to above 7 with sodium hydroxide, effectively precipitating the chromium as Cr(OH)3. Ferrous sulfate is also an effective flocculating agent, eliminating the need for further addition of flocculants.

The nickel rinsewaters were treated with sodium hydrox- ide to obtain a pH between 7 and 9 where the nickel hy- droxide solubility is lowest, then with ferric chloride (FeC13.6H20) in excess (3.23 g I - ' ) to promote floccu- lation and again with sodium hydroxide to readjust pH to the desired value.

The copper and zinc rinsewaters contained, respectively, soluble copper and zinc cyanide complexes requiring des- truction prior to metal hydroxide precipitation. Six percent sodium hypochlorite, applied at a level of 16-21 ml I- ' of plating rinsewater, was used for this purpose. The two-step oxidation process yields cyanates during the first stage at pH 9-11 and nitrogen and bicarbonates during the second stage at pH 8.5. Reaction times for each stage were from 30 to 60min (US EPA, 1979a). Following this cyanide des- truction, the pH was adjusted with sodium hydroxide to provide minimum solubility of the metal hydroxide. Next, ferric chloride (FeO3.6HzO) was added in excess

(1.6-2.5 g I-'), followed by sodium hydroxide for pH re- adjustment. Reaction times for precipitation and floccu- lation were in all cases 30 min for each operation.

Runs with the ultrafiltrution pilot utiir

A schematic diagram of the ultrafiltration pilot unit is shown in Fig. 1. The in-line strainer includes a bleached cheese cloth or similar material.

The ultrafiltration stack is of the plate-and-frame type. I t contains three passes in series, each including two mem- branes of 214 cm2 area, giving a total area of 1284 cm2 (Chrisleiisen Cyr Plaumann, 1981). Chomical pretreatment took place in the feed tank using the agitator to enhance mixing, precipitation and flocculixion. The volume of treated rinsewater in the feed tank decreased during the run as the concentrate was returned to the feed tank and the permeate was directed to a separate container. Per- meate flux rates were obtained at a constant inlet pressure of 241 kN m-' (35 psig) and constant outlet pressure of 34.5 kN m-2 (5 psig). Flux rates were determined by timing the flow of permeate into a graduated cylinder. A constant temperature of 25 + 1°C was maintained by a heating/ cooling coil (not shown) immersed in the feed solution.

The concentration of metals in the permeate and in the feed tank waste was measured using a Perkin-Elmer Model 305A atomic absorption spectrophotometer and standard techniques (APHA, 1976; Perkin-Elmer Corp., 1973).

RESULTS AND DISCUSSION

Theoreticul and experimentul solubilities The solubility of chromium, nickel, copper and zinc

in the presence of hydroxyl ion as the major anion can be calculated based on the solubility product con- stants and formation constants for various hydroxy complexes.

The reactions of interest for trivalent chromium, along with the definitions of appropriate equilibrium constants, are listed in Table I (Sillen & Mariell, 1964). Based on this information, the total concen- tration of soluble Cr(ll1) i n the prcsci~cc of chronhi i i Iiydroxidc prcuipiltrtc cui1 be cxprcescd us

S (mol 1 - of Cr(ll1))

CONCENTRATE ' FLOWMETER . A n l T A T n r ) . -. . . . . -. . I

PRESSURE - PRESSURE

VORTEX BREAKER TANK

. . '3

Fig. 1. Schematic diagram of the ultrafiltration pilot unit.

Table 1. Chemical reactions relevant to the solubility of chromium(Il1)

Reaction Equilibrium Definition of

constant derived constnnt

The concentrations given by equation (1) are plotted as a function of pH in Fig. 2, using the numerical valus of the various constants from Table 4.

figure, and it is seen that the addition of ferric chlor- ide as a flocculating agent is essential for the achieve- ment of low solubilities. The points a t pH 7.6 and 9.2

Actual concentrations of soluble chromium are also indicated in Fig. 2. Experimental points were obtained with a single batch of wastewater, first for the highest pH and then for decreasing values of pH. Except for the lowest point a t log[Cr(III)] = -5.5, (pH = 4.3) there is fair agreement with the curve. Note, however, that Log[Cr(III)] = -2.92 is the maximal value for pH = 4.3, based on the total chro- mium concentration of 63 mg I - ' of the waste in the feed tank. The reason for the deviating point may be a longer time constant for dissolution than for precipi- tation. This difference is accentuated when an aged (2-day-old) precipitate is to be redissolved (Stumm & Morgan, 1970).

The agreement with calculated solubilities indicatcs that hexavalent chromium has been etrcctively reduced to trivalent chromium (US EPA, 1979a):

3FeZ" + 00:- + 4 H 2 0 4 3Fe3+ + Cr3+

+ 8 0 H - . (2)

The solubility of nickel is governed by the reactions given in Table 2. Based on these reactions, the total concentration of soluble nickel is given by the follow- ing expression :

S (mol 1 - I of Ni)

were obtained for the same batch and in the given order. The much smaller solubility at pH 9.2 is in accordance with a small rate constant for precipi- tation compared to that for dissolution. The experi- mental solubilities using ferric chloride are consistent with those obtained theoretically with partially aged precipitate.

The solubility diagram for copper (Fig. 4) reflects the three forms of Cu(I1) precipitate: inactive CU(OH)~, active CuO and inactive CuO (Feitknecht 8t Schindler, 1964). Because of the absence of hydroxy

1

The levels obtained, using numerical values of con- pn

stants fm'l'l Table 4, are Plotted as a f~nc t ion of PH in Fig. 3. Experimental data are also included in this

Fig. 2. Theoretical solubility curve for Cr(I1l) as a function of pH with indication of experimental data.

Table 2. Chemical reactions relevant to the solubility of nickel and zinc

Equilibrium Definition of Reaction* constant derived constant

KS0 'I Me(OH),(s) e Me2' + 20H- Me(OH),(s) F? Me(0H)' + OH- K,I PI = K,IKS' Me(OH),(s) e Me(0H); K,z P z = K,zKG'

Me(OH),(s) + 20H- Me(0H):- Ka4 P4 K,&G' Me(OH),(s) + OH- e Me(0H); K-3 83 = K"3KSL

* Me is substituted for Ni or Zn.

132

6

4 -

2 -

ERIK R. CHRISTENSEN and JOHN T. DELWICHE

I I I I I

.- a 4 e I i o i a 14

vn

Fig. 3. Theoretical solubility curves for Ni as a function of pH in the presence of fresh and aged precipitate. Experi-

mental data are also shown.

complexes, the curve'for the Cu(1) precipitate, Cu20, is a straight line. The solubility curves have been cal- culated on the basis of the relevant chemical reactions and equilibrium constants (Tables 3 and 4). The con- centration of soluble copper(I1) is given by

S (mol 1 - of Cu(I1))

+ B J 0 H - l 3 + B4[OH-I4). (4)

The experimental data (Fig. 4) indicates that the addition of sodium hypochlorite at 984 mg I - ' gives substantially lower solubilities than a level of 32 mg I-'. The net reaction for the destruction of cya- nide with hypochlorite proceeds as follows (US EPA, 1979a):

2NaCN + 5NaOCI + H 2 0 3 5NaCl+ N2

+ 2NaHC0,. (5)

At higher concentrations of hypochlorite and follow- ing lower levels of cyanide, the formation of copper cyanide complexes becomes less likely. However, the hypochlorite concentration of 984 mg I- ' is appar- ently not high enough to prevent stable copper cyan- ide complexes (Table 5) from becoming dominant, as indicated by the large gap between actual data and theoretical curves (Fig. 4). The cyanide concentration in the untreated rinsewater was 12gl -I as sodium cyanide (Hooks, 1980).

The calculation of solubility curves for zinc (Fig. 5) is similar to that for nickel. Chemical reactions and equilibrium constants are given in Tables 2 and 4.

The total concentration of zinc in solution is given by :

S(mo1 I - ' ofZn)

=- K w (1 + BJOH-3 + P2[OH-12 [OH-]'

Actual data indicate that the addition of ferric chloride enhances the formation of particulates and therefore, lowers the solubility. There is good agree- ment between experiment and theory for fresh pre- cipitate. The addition of sodium hypochlorite in the amount of 1.35 g I - ' is in substoichiometric quantity relative to the prevention of cyanide complexation of the available zinc, based on a sodium cyanide content of ihc unlrcntcd riiibcwiitcr of 1s y I - ' (Iiooks, 1980). NUVGIIIIUIU~~N. IIm rriiiuiint ol' I i y p ~ ~ c l ~ I d t u uppuui I irr be sutticient to diminish the contribution of the rela- tively unstable zinc cyanide complexes (Table 5) to the total soluble zinc so that the zinc solubility is determined mainly by the hydroxyl ions.

All experimental solubility data are summarized in Table 6.

Perjormunce of the ul&#tration runs

Hydraulic parameters and initial water quality par- ameters for all ultrafiltration runs are given in Table 7. It is clear from this table that chemical p r e treatment is crucial for the achievement of low metal concentrations in the permeate. The only exception from this rule is zinc (run 18) for which there is a relatively high initial removal ratio Ro = 1 - c,,,,/c, = 0.69 without chemical pretreat- ment. This may be caused by an association of zinc

\\ I1

11 U n

11 11

I1

/I

given

(6)

erric and [ree- pre- the

itity n of tent 80). i to ela- ' to 1 is

I in

ar- in

r e tal on 're io It- nc

I

I

Ultrafiltration of electroplating rinsewaters 133

Table 3. Chemical reactions relevant to the solubility of copper

state Reaction constant derived constant Oxidation Equilibrium Definition of

KS0 CUO(S) + H20 F? CU" + 20H- CU" + H20 3 Cu(OH)+ + H+ * K , pi =:

Ks3 B 3 = K83KG' CuO(s) + H 2 0 t 20H- e Cu(0H):- K.4 P4 K.dK10'

K s 2 2 Bo = K.22KG1 K 10

2

CUO(S) + H20 + OH- e Cu(0H);

~CUO(S) + 2H20 F? 20H- + CU,(OH):+ 1 CUZO(S) + H 2 0 ~ C U + + 20H-

Table 4. Equilibrium constants used in solubility calculations

Log of equilibrium constant. pH of minimum

Precipitate Kso Ksi Ksz Ks3 K,4 *K , I Kszz * K , *K2 *& solubility

Cr(OH), (fresh) - 30.0 -0.4 8.0 -6.2 -4.0 8.2 -14.7 -10.9 -3.9 -1.7 9.3-10.5

(aged! -17.2 -13.4 -6.4 -4.2 9.3-10.5 Ni(OH), (fresh)

CU(OH)~ (inactive) - 18.8 CuO (active) -19.7 CuO (inactive) -20.5

Zn(OH), (fresh)

-3.6 -2.7 -20.6 -8.0 9.4 -4.5 -3.6 -22.2 -8.0 9.4 -5.3 -4.4 -23.8 -8.0 9.4

CU,O - 14.7 -16.0 -11.6 -4.5 -1.8 -0.5 8.9-1 0.3

(aged) -17.0 -12.6 -5.5 -2.8 -1.5 8.9-10.3

* From Sillen & Martell (1964). Feitknecht and Schindler (1963). and Patterson et a/. (1977a).

with particulates which are effectively removed by ultrafiltration.

The runs of special interest are those for which the permeate concentration cpo is minimum, since the aim of this work is to remove metals from the rinsewater. These runs are No. 3 for Cr with c,, = 0.17mgI-', No. 12 for Ni with cpo = 0.26mgI-', No. 16 for Cu with cp0 = 0.30mgl-I and No. 20 for Zn with cn0 = 1.84mgI-'.

Another important parameter is the water recovery q, that is the fraction of the feed stream which per- meates the membranes. A high water recovery is desirable because it provides the maximum volume of filtered wastewater in a given time and with a given pumping power. For the runs identified &ove as pro- ducing minimum permeate concentrations, the high- est water recoveries occur for nickel with a 0.20pm membrane (q = 24%) and chromium with a 0.80pm

T&!e 5. Stsbl!i:y c~fis!afi!s of Copijii eiid zinc ej;mide complexes

Oxidation Stability Metal state Complex* constant?

membrane (q = 10%). By comparison, the recoveries are only 6.5% for copper and 3.7% for zinc, both with a 0.45 pm membrane.

During the ultrafiltration runs, the configuration of the pilot unit was as shown in Fig. 1. Thus, the feed flow Q is divided into a permeate flow qQ, which is directed to a separate container and a concentrate flow (1 - q)Q which is recycled to the feed tank. The rate of decrease of the volume of waste in the feed

ERIK R. CHKETENSEN and JOHN T. DELWICHE

Table 6. Experimental solubility data

Initial permeate

NaOCl FeCI,.6H20 Concentration Log [Me]* Metal (mgl-I ) pH (g1-I) c , , ~ (mgl- ' ) [Me](moll-')

Cr( I I I) 10.6 1.04 - 4.70 8.9 0.87 - 4.78 (4.3) 0.17 , - 5.49

Ni 10.0 37.5 - 3.19 12.0 32.5 - 3.26 11.1 25.8 - 3.36 7.6 3.23 5.09 - 4.06 9.2 3.23 0.26 - 5.35

cu 32 7.8 1.49 25.5 - 3.40 32 8.2 1.49 24.8 - 3.41 984 8.8 2.45 0.30 - 5.33 984 10.3 2.45 0.33 - 5.28

Zn 1350 11.0 3 7.10 - 3.96 1350 8.6 1.58 . 2.43 - 4.43 1350 10.6 1.58 1.84 -4.55

* h-le IS subrtitutrd for Cr(ll1). NI, Cu or Zn.

tank is, therefore, equal to the permeate flow qQ. The metal concentration c,(t) in the permeate is expected to remain constant with time t because the solubility under chemical equilibrium is independent of the rela- tive amount of the solid phase.

A mass balance for a given metal during the filtra- tion of a batch of rinsewater must reflect that the total mass of metal in the feed tank at any time, plus the mass of metal lost through the permeate slream, should be constant and equal to the initial total mass of the metal in the feed tank. Assuming constant metal concentration cp(t) = cp in the permeate, this condition can be written as follows:

(7) c(t)(V, - v Q ~ ) + c,,qQt = coV0,

where c(r) (mg I - ' ) is the metal concentration in the feed solution and Vo(l) is the initial feed volume. Introducing the initial removal ratio Ro as defined above and the relative volume filtered a = qQtV, I , the following expression is obtained for the relative feed concentration:

Values of the relative feed concentration based on this equation are plotted in Fig. 6 as a function of a. Ex- perimental data for the rclaiivp fced and pcrmea!s concentrations in run Nos. 3, 12, 16 and 20 are also shown in this figure.

From Fig. 6, the expected constancy of the per- meate concentration as a function of volume filtered is essentially confirmed, except for an apparent initial IS%, drup Tor nickel. With regard to the rclutivo feed concentrution, tlic cxpcriaiciitul dirtii follow tlic tlicor- etical curves well for relative volumes up to about 0.3. Above this value, there is a discrepancy indicating that the strainer may act as a trap of precipitated metals (Fig. 1).

The achieved effluent levels are compared to pre- cipitation technology standards and pretreatment standards in Table 8. As can be seen from this table, the achieved levels of Cr(T), Ni and Cu are substan- tially lower than the precipitation technology stan- dards. The reason for this may be the better solids separation obtained with ultrafiltration than with conventional technology using gravity settling and sand filtration. The somewhat higher level of zinc obtained in this "work (1.84 vs 1.0mgI-I) could be caused by incomplete hypochlorite oxidation of cyan- ides. The achieved levels conform in all cases to cur- rent pretreatment standards. In order to more fully evaluate the potential of ultrafiltration, the long-term membrane performance should also be considered.

CONCLUSIONS

Chromium, nickel, copper and zinc can be effec- tively removed from electroplating rinsewaters by precipitation, flocculation, and ultrafiltration. Prior to hydroxide precipitation, hexavalent chromium is reduced to the trivalent state, and cyanide is removed from copper and zinc rinsewaters. I Minimum metal concentrations in the permeate from separate batches of Cr, Ni, Cu, and Zn rinsewaters were found to be,

0.30 mg I - ' Cu and 1.84 mg I-IZn. The solubilities as a function of pH for Cr(llI), Ni,

and Zn were found to be in good agreement with those calculated on the basis of known chemical reac- tions governing the concentrutions of soluble species, ond the ussocinicil cquilibrirrtii coiisteiiis, iisstitiiitig frchli uiid iryetl p w i p i t u t v , iior cuppor, tlic nciucll solubililies are a1 least two orders, of magnitude higher than the theoretical ones. This discrepancy may be caused by incomplete cyanide removal and following formation of stable, soluble copper cyanide

rcspec:i;,e!y, 0.17 q l - I CjjTj, 6.26 nlg i - ! xi ,

r ::-

I -

. ,,

Table 7. Summary of results

Initial Initial Chemical addition; Initial feed feed Feed Water Initial permeate

conc. volume Pore 50% flowrate recovery Permeate removal conc. Run Date of co V, size NaOCl FeS04.7H20 NaOH* FeCI,.6H20 Q tl flux Q ratio CPO

Metal NO. run (mgl-I) (1.) (pm) (mgl-') (g1-l) (m1l-l) fg1-l) pH (1min-I) (%) (1m-2hP-1 1 RO (mg1-I)

Cr 1 2/24/80 84.0 86.1 0.80 2.23 0.37 10.6 13.75 11.6 749 0.9876 1.04 c_ 669 0.9878 0.87 c

3 2/29/80 63.0 86.1 0.80 2.23 0.14 (4.3) 12.63 10.4 614 0.9973 0.17 a 4 3/6/80 89.2 94.6 0.80 5.5 13.09 7.5 460 0 89.2 * 5 3/7/80 82.5 94.6 0.80 0.21 10.9 13.07 7.4 450 0.0303 80.0 2

2 2/27/80 71.3 86.1 0.80 2.23 0.33 8.9 12.71 11.2

P

5. 0, 0,

7 1/15/80 45.5 47.3 0.002 1.06 10.0 11.95 11.3 630 0.1758 37.5 z!

8 1/16/80 39.5 47.3 0.002 1.86 12.0 11.95 11.3 632 0.1772 32.5 9 1/24/80 33.7 47.3 0.002 1.86 11.1 11.62 12.0 655 0.2344 25.8 0

- 1 Ni 6 1/14/80 44.3 47.3 0.002 7.5 11.65 12.3 670 0 44.3

10 5/13/80 396 94.6 0.45 7.2 13.21 2.6 159 0 396 9, 11 5/15/80 378 94.6 0.20 2.39 3.23 7.6 14.13 25.0 1652 0.9865 5.09 g. 12 5/20/80 339 94.6 0.20 2.90 3.23 9.2 11.99 24.2 1359 0.9992 0.26 m

Cu 13 3/20/80 28.1 94.6 0.80 10.0 13.06 4.4 268 0.0071 27.9 B 501 0.2500 25.5 2

1.

14 4/06/80 34.0 94.6 0.80 32 0.79 1.49 7.8 13.37 8.0 15 4/08/80 33.1 94.6 0.80 32 0.79 1.49 8.2 13.71 10.3 660 0.2508 24.8 16 4/14/80 30.6 94.6 0.45 984 1.45 245 8.8 12.75 6.5 388 0.9902 0.30 17 4/17/80 29.8 94.6 0.45 984 1.61 145 10.3 12.37 6.7 385 0.9889 0.33

Zn 18 4/24/80 23.1 94.6 0.45 11.0 13.60 4.0 254 0.6926 7.10 19 5/04/80 33.6 89.9 0.45 1350 0.56 1.58 8.6 12.93 4.9 296 0.9277 2.43 20 5/04/80 30.2 89.9 0.45 1350 0.78 1.58 10.6 13.16 3.7 226 0.9391 1.84

2 v)

* Net amount after addition of NaOH and HCI.

736

al d a 3

ERIK R. CHRISTENSEN and JOHN T. DELWICHE

I I I I I I I - 3.50 -

o NI, run No. 12 p Cu, run No. 16

-

- (a)

2.00 - -

1.50 - -

-

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Relatlve volume filtered, '7 QtlVo

1 I 1 I I I I I 1

0.80

'*0°

0 i 0.1 0.2 0.3 0.4 0.6 O._B 0.7

Relatlve volume flltered, OL "'1 Qt/Vo

Fig. 6. Relative (a) feed and (b) permeate concentration as a function of the relative volume filtered. Theoretical curves are compared to experimental data.

complexes. Less stable copper cyanide complexes are formed at higher temperatures (Sillen & Martell, 1964), and increasing the temperature may, therefore, be one way of lowering the copper solubility besides the more obvious one of increasing the amount of oxidizing agent, e.g. sodium hypochlorite, to obtain a more effective cyanide destruction.

A simple mass balance model assuming concentrate recycle and constant metal concentration in the per- meate, is able to predict the relative feed concen- tration as a function of the relative volume filtered during an ultrafiltration run. The assumption of con- stant metal concentration in the permeate has proved to be correct and, therefore, confirms that the concen-

Table 8. Comparison of achieved effluent levels with precipitation technology standards and pretreatment standards

Prerrearmenr standards? Precipitation "-')

Achieved technology levels standards* 1-day 30-day-average

Metal (mg I - ' ) (mg I - ' ) maximum maximum ~~

Cr(T) 0.17 1 .o 7.0 2.5 Ni 0.26 1 .o 4.1 1.8 cu 0.30 0.5 4.5 1.8 Zn 1.84 1 .o 4.2 1.8

* Patterson (1977a). t US EPA (1979b).

(" Ultrafiltration of electroplating rinsewaters 737

trations of metal in solution are governed by chemical equilibrium with the solid phase.

Water recoveries are strongly dependent on the removed, and were found to be 24% for Ni with pm membrane, 10% for Gr with a 0.80 pm mem- , 6.5% for Cu and 3.7% for Zn, both with a

REFERENCES

(1976) Standard Merhods for rhe Examination of rand Wasrewater, 14th Edition. Washington DC.

la, S. & Jump R. (1977) Metal recovery makes good nse. Enoir. Sci. Technol. 11, 752-755.

en E, R. & Plaumann K. W. (1981) Water reuse: tion of industrial and municipal wastewaters. J .

. Control Fed. 53, 1206-1212. . & Schindler P. (1963) Solubility constants of

metal hydroxide salts in aqueous solution. Pure App l . Chem. 6 , 130-199.

Crepor H. P. & Grepor C. D. (19781 Synthetic-membrane tcclinolopy. Sr i Atti 239, I 1 2 128.

Hcrtrks E. (lO7H) Pcrwiiel communicnlion. Maria J. (Ed.) (1"N M e t d Finidtiwq: 4,Wt G i h / e / ~ ~ i o k -

Directory Issue. Metal & Plastics Publications, New Jersey.

Patterson J. W. (1977a) Technical iniquities in effluent limi- tations guidelines. J . Wac. Pollut. Control Fed. 49, 15861590.

Patterson J. W., Allen H. E. & Scala J. J. (1977b) Carbo- nate precipitation for heavy metals pollutants. J. War. Pollur. Conrrol Fed 49, 2397-2410.

Perkin-Elmer Corp. (1973) fnsrrucrion Manual for Atomic Absorption Spectrophotometer, Model 305A. Norwalk, CT.

Porter M. C., Schratter P. & Rigopulos P. N. (1971) Bypro- duct recovery by ultrafiltration. Ind. War. Engng 8,

Sillen L. G. & Martell A. E. (1964) Stability Constants of Metal-Ion Complexes. Special Publication No. 17. The Chemical Society, London, UK.

Stumm W. & Morgan J. J. (1970) Aquatic Chemistry, 583 pp. Wiley, New York.

U.S. Environmental Protection Agency (1979a) Environ- mental Pollution Control Alternatives: Economics of Wastewater Treatment Alternatives for the Electropla- ting Industry. EPA 625/5-79-016, Industrial Environ- mental Research Laboratory, Cincinnati, OH.

IJ. S. Environmental Protection Agency (1979h) Effluent guidelines iind atnndsrds: clectroplriting point source category; pretreatment standards for existing sources. Fed. R e g . 44, 175.

18-24.