b REMOVING RADIONUCLIDES

Exhausting and regenerating resin for uranium removal

As expected, lowering the pH adversely affected uranium

removal by anion resin-but at a lower than anticipated pH.

Zhihe Zhang and Dennis A. Clifford

he US Environmental Protection 1) recently proposed a maximum cant

t 1

ACL) ot 20 pg/L and a maximum con )a1 (MCLG) of zero pCi/L for uranium

of concerns about its association with kidney disease and cancer. Based on the results of a national survey, it was estimated that 1,500 community water sup- plies in the United States could be out of compli- ance and would have to reduce uranium con- centrations to the pro- posed level. l Although not well documented, several small commu- nity water supplies have uranium concentrations in the high micrograms-

Agency .aminant aminant because

228 JOURNAL AWWA

Houston-US Environmental Pm mobile laboratory was used at

to study possible nium removal.

rbonate anions to form stable, (or both) uranyl car-

most significant species in drinking water supplies.4 Under mildly acidic conditions of pH 5.0-6.5, the principal uranyl carbonate species is UO,CO,a, whereas under neutral and alkaline conditions the principal species are U02(C03),2- and UO, (CO,) a4-. Uranyl hydroxy complexes such as U020H+ and (UO2)3(OH)s+ are also formed, but generally in small percentages unless at high temperature or in carbonate- depleted water at pH >lO.s Figure 1 shows the distribution of typical uranium complexes as a function of pH in aerobic groundwater at a carbon dioxide partial pressure of 10-2 atm.4 The reported stability constants of var- ious complexes of uranium are given in Table 1 .6-l Because of their high solubility and con- sequent dissolution from minerals and mine tailings, uranium compounds are present in both groundwaters and surface waters.

per-litre range-even up to the milligrams-per-litre range.2 The abandoned community water supply well used for this research in the Chimney Hill subdivi- sion, 25 miles northwest of downtown Houston, Texas, was considered typical of a groundwater highly con- taminated with uranium. Its uranium concentration was normally in the range 1 lo-125 pg/L.

This article is one of several generated from the results of the USEPA-funded 20-month bench- and pilot-scale field study in the Chimney Hill subdivi- sion on possible techniques for uranium removal. As detailed in the article on page 2 14,3 the University of Houston-USEPA mobile drinking water treatment research facility was used for the field study.

Uranium chemistry and radioactivity

Uranium is not found in the elemen- tal form because of its strong reactivity. It has four oxidation states-III, IV, V, and VI-with the IV and VI states being most stable. The high affinity of uranium for oxygen results in the formation of uranyl ion, UO,2+, which is stable under acidic conditions (pH ~5.0). The uranyl ion read- ily combines with ligands such as Cl-, NO,-, SOd2-, HPOa2-, and C032- to form stable complexes depending on the pre- vailing water chemistry. In the near-neu- tral pH range, UO, 2+ combines with bicar-

The three naturally occurring uranium isotopes, with their corresponding percentage occurrences and half-lives, are as follows: 238U (99.2745 percent, t, = 4.468 x lo9 years), 235U (0.7200 percent, t, = 7.037 x lo8 years), and 234U (0.0055 percent, t, = 2.454 x 10s years).ll When these isotopes are present in a water at these percentages, the relationship between uranium radioactivity and concentration is 0.69 pCi/pg.3 A slightly different specific activity-O.67 pCi/pg-has been reported by others.12,13 Caution must be exercised when using an average specific activity because uranium isotopes in groundwater are not always found in their reported natural abundance ratios because of varying geochemical conditions.12

TABLE 1 Stability constants of uranium compounds occurring in groundwater

APRIL 1994 229

t-b@ P Summary of runs used to remove uranium

m

nium exhaustion on regen- eration, and

l to predict the effects of sulfate and feed uranium concentrations on run length.

Materials and methods Feedwater. In addition to

1 lo-125 ug/L uranium, the Chimney Hill water con- tained 20-25 pCi/L radium. Details of the feedwater com- position, pretreatment facil- ity, and the flow schematic for the mobile laboratory have been described in the companion article.3

Because the well was out of service, it was pumped occa- sionally to fill a 2 1 O,OOO-gal storage tank, which provided water for the experiments. The observed variations in feed- water uranium concentration were attributed to its adsorp- tion onto the traces of hydrous iron and manganese oxide pre- cipitates that formed or were stirred up each time the tank was filled. Although the ura- nium concentration was nor- mally in the range of 11 O-l 2 5 pg/L, it once reached a low 67 ug/L. When low-pH water was tested, the feedwater was first acidified to the required pH range in the feed tank with 0.2 N hydrochloric acid (HCl) before being pumped through the resin beds.

Resins. The type of resin used was not a variable in this research because previous studies had shown the typical polystyrene quatemary amine SBA resins are adequate for uranium remova12,17 Among the many possible anion resins, the one* chosen had successfully been used in full-

i

sElG#?t 1 Distribution of typical uranyl complexes as a function of PH for

: ,; ;;! ho, = 1W2 atm, 25W, and Chr = 2.4 pg/L4

scale uranium removal. la This macroporous resin con- sists of a polystyrene-DVB matrix, with fixed quater- nary ammonium-N(CH3)3+-exchange groups in the Cl- form. Its advertised capacity is 1 meq/mL.

Column studies. Generally, the downflow fixed- bed column runs were conducted at ambient tem- perature (20-24C) in l- and 2-in.-diameter glass columns with graded-gravel support. Eight columns, consisting of pure SBA resin, were tested in once- through mode. Cyclic runs to predetermined BVs were aimed at studying (1) the removal of uranium

and (2) the regeneration of the partially uranium- exhausted resin with a variety of regenerants. Once- through exhaustions were performed in an attempt to characterize the capacity of the columns for remov- ing uranium under different pH conditions. The par- tially uranium-exhausted resins were used later dur- ing the regeneration studies, which were generally carried out in 50-mL burettes. The details of the total BVs and run conditions, including the empty bed

*lonac A-642, Sybron Chemical Co., Birmingham, N.J.

APRIL 1994 231

BVs of Concurrent downflow regeneration

was employed for all the regeneration runs, for which the regenerant EBCT was always 10 min. A regeneration was typically fol- lowed by slow and fast rinses to displace desorbed uranium. EBCTs for the slow and fast rinses were 10 and 5 min, respectively. During regeneration, the water level was maintained at 6 in. above the resin.

For the so-called complete regenera- tions, six BVs of 1.7 N NaCl (37 lb/cu ft) were employed to give a regeneration stoichiometry of 10.2 eq Cl-/eq resin. For other regenerations, the appropriate num- ber of BVs of 0.5, 0.8, 1.0, 1.3, 1.7, 2.0, 3.0, or 4.0 N NaCl was used to give a mass loading of salt corresponding to 4.0 or 6.0 eq Cl-/eq resin.

Some experiments determined the influence of high pH on uranium recov-

ery efficiency. In these experi- . . . . .._. n of virgin ments, NaCl was mixed in a

2:l molar ratio (NaCl:base) with NaOH, NaHCO,, or Na,CO,. Each of these mix- tures was tested for regenera- tion efficiency at various con- centration levels with the total amount of regenerant un- .: i _. , j changed. Finally, 1 N HCl was

::.,.A .--, ., 1 ;, tested as a regenerant and :. : ,:

3 compared with the NaCl and

$., ; ,,

s NaCl plus base regenerations.

v. Standards and reagents. Uranium standards were

6 obtained from three sources, . the USEPA Environmental Monitoring Systems Labora- tory (EMSL) in Las Vegas, Nev., and two chemical com-

c a -9 I ,% 2&o 30,ioo

5 panics,* and were used suc- cessfully to cross-check one

contact time (EBCT), for all the exhaustion runs are summarized in Table 2.

For purposes of tracking data, samples, and analy- ses, the runs were numbered as follows: E designates exhaustion, R designates regeneration, and the number after the decimal point designates the cycle number. For example, run E 11.1 is the first exhaus- tion during cyclic run 11, and R 11.1 designates the first regeneration during cyclic run 11.

Regeneration. After the termination of exhaus- tion runs E 4.1, E 11.1, and E27.1 of the virgin resin, the resin was removed from the column and well mixed. The mixed resin was then divided into several 25-mL portions, which were packed into 50-mL burettes for the regeneration study.

another. All the chemicals used for the pilot runs and analyses were analytical reagent grade chemicals.

Uranium analyses. Uranium concentrations were determined using the analyzer+ on loan from USEPA, which had used the instrument and the associated laser fluorescent technique for a national radionu- elides survey.12 The principle of the method is described in the accompanying article.3 This uranium analyzer was standardized at least twice per analysis run using 2-, 20-, 120-, or 200-pg/L standards, depending on the expected range of sample concentrations. The pre- cision of uranium analyses was improved by always running duplicates or triplicates and taking the aver- age. The relative standard deviation for the uranium

*Mallmckrodt Chemical, Inc., Chestcrflcld, MO.; and Aldrich Chemcal Inc., Milwaukee, WE.

tUA-3, Scmtrex Ltd , Concord, Ont , Canada

232 JOURNAL AWWA

method was 5.4 percent as determined from 55 pairs of water samples that exhibited an average concen- tration of 5.6 pg/L.

To further ensure accuracy, uranium standard solu- tions and some actual test samples ranging from a few micrograms per litre to lo6 pg/L uranium were also cross-checked using the inductively coupled plasma (ICP)-mass spectrometry (MS) method at the USEPA Risk Reduction Engineering Laboratory in Cincinnati, Ohio. Good agreement was obtained between the laser fluorescent technique and the ICP-MS method.

Other analyses. Total hardness, total alkalinity, total dissolved solids, and pH mea- surements were performed as specified in the 16th edi- tion of Standard Mefhods.i9

Results and discussion Resin capacity and ura

nium leakage. Two columns containing 12- and 27-m. bed depths of virgin SBA resin operated to 30,000 and 41,000 BV throughputs, respectively. These natural-pa runs exhibited excellent ura- nium removal; at pH 7.6-8.2 the effluent uranium concen- tration was always CO. 1 pg/L, i.e., >99 percent removal.

The effluent history for run E4.1 is shown in Figure 2. This 2-in.-diameter, 20-in.- bed-depth column exhibited excellent uranium removal. It operated continuously for 478 days for a total throughput of 302,000 BVs at pH 7.6-8.2. Even after 300,000 BVs, the effluent uranium concentra- tion was still

The 1Pff wide x 13-ft lo /aboratory (rignt) uranium and radium

Houston-area supply. Feed p4 top) delivered raw

exchange columns, and rignt, bottom) mo

Effect of pH on uraniud Although the natural pH I water was 7.6-8.2, several runs were conducted at pH 5.6-6.0 and pH 4.2-4.5 to investigate the effects of lower-pa feedwater on uranium removal. A substantial decrease in the resins capacity for uranium removal was expected at lower pH because the charge on the uranyl carbonate com- plex decreases with decreasing pH as does the carbonate concentration in the feedwater. However, when pH was decreased-by HCI addition-from 7.6-8.2 to 5.6-6.0, uranium removal efficiency was almost unchanged. If the pH was further decreased to 4.2-4.5, however, serious uranium leakage occurred from the very beginning of the run. The overall relationship between uranium leakage and pH is illustrated in Figure 3.

pH 5.8. Figure 4 shows the effluent history of run 31 at pH 5.8, which lasted for nearly 20 days for a total of 26,300 BVs of throughput. The highest uranium leakage observed during the entire run was ~0.5 ug/L. As observed during the pH 8.0 runs, excel- lent uranium removal (~99 percent) was achieved at pH 5.8.

Runs 28 and 29 were conducted in two pH stages. First the resin was fed pH 8.0 groundwater for the first 41,000 BVs, then the pH was lowered to 5.8 as the runs continued. Figure 5 demonstrates the results from run 29.1, which were identical to those of run 28.1. Lowering the pH of a partially exhausted resin to 5.8 produced an insignificant increase in uranium leakage to approximately 0.5 from CO. 1 pg/L.

The excellent pH 5.8 uranium removal results indicate that, as far as leakage is concerned, uranium removal is nearly as efficient at pH 5.8 as it is at pH 8.0. This unexpected performance at an acidic pH requires some explanation. At pH 5.8, the dominant uranium species in the aqueous system in the absence of anion resin is the zero-charged UO,( CO,)O complex (Figure 1). Nevertheless, as the following discussion demonstrates, uranium removal can still be explained as the uptake of a tetravalent UO,( C03)a4- complex in exchange for four Cl- ions.

It is theorized that the direct formation of charged U0,(C03)z4- from uncharged IJO,CO,a occurs within the ion exchanger because of the resins high affinity

for polyvalent anions and the high stability constant of the tricarbonate uranium complex. The suggested exchange reactions are

4 RCl + 4 HCO,- - 4 RHCO, + 4 Cl- (3)

4 RHCO,+UO,CO,a-R4U0,(C03), +2 H2C03 (ad t4)

Eq 3 is the conversion of four resin sites to the bicarbonate form by simple ion exchange for chloride. In Eq 4, these sites are converted to the much pre- ferred uranyl tricarbonate (R,U02(C03)3) form with the production of carbonic acid [H,CO, (aq)]. The resulting lowering of pH by the HzCO3 (aq) produced is insignificant because of the very low concentra- tion (0.0005 mM) of U02COso in the feedwater com- pared with the total carbonate concentration ( CT,co3) of the carbonate buffer system-3.00 mM.

Adding Eqs 3 and 4 yields the net ion exchange reaction:

4 RCl + UO,CO,a + 4 HCO,- = R,UO,(CO,), + 2 H2C03 W + 4 Cl- (5)

Eq 5 is, effectively, the exchange of a neutral U02COso molecule for chloride ions.

The production of U02(C03)24- in the resin can also be attributed to the production of C032- in the resin from the abundant bicarbonate ions in the water. In this mechanism, the proposed reactions are as follows:

2 RCl + HCO,-- R,CO, + 2 Cl- + H+ (6)

224 JOURNAL AWA

ma1 pH of 8.0, the HCO,- concentration was 2.92 mmol/L; at the adjusted pH value of 5.8 and 4.2, HCO,- was still significant at 0.66 and 0.026 mmol/L, respectively. Considering that a uranium concentration of 120 pg/L translates to 0.0005 mmol/L UO, (CO,)O or UOz2+, much more than the required stoichiometric amount of HCO,- was available in the water to react with the resin to produce the additional CO,z- needed to progressively complex the ura- nium according to Eq 5 or a combination of Eqs 6 and 8.

pH 4.3. Run 33.1, as presented in Fig- ure 6, was conducted at an adjusted feed pH of 4.3 f 0.1 for 340 h with a total throughput of 6,800 BVs. Lowering the feed pH to 4.3 with HCI drastically impaired the uranium removal perfor- mance of the anion bed. Only about 50 percent uranium removal was achieved right from the start of the run. Significant uranium breakthrough began :at about 4,000 BVs and was complete at about ,*, .,,. I_ ..- 6,000 BVs when the column was appar- ently in equilibrium with the feedwater and no uranium was being removed. This early breakthrough is in sharp contrast to the excellent performance at pH 8.0,

In Eq 6, two resin sites, which much prefer one when the macroporous resin was operated to 302,000 divalent ion to two monovalent ions, convert bicar- BVs, at which point the uranium leakage was still bonate to carbonate with the production of a pro- only 5 percent of the feed uranium concentration. ton. Like Eq 4, Eq 6 disturbs the normal uranyl car- Although poor, this 50 percent uranium removal bonate equilibrium described in Figure 1. Again, the was better than expected for an anion resin, based on system is buffered against significant pH reduction the cationic nature of the UOz2+ present in the feed- because H+ reacts with the abundant HCO,- ions to water at pH 4.3. produce H,CO, (aq) as indicated in Eq 7. Here again, the explanation for uranium uptake is

based on the known high affinity of strong base anion HCO,- + H+ - H,CO, (aq) (7) resins for divalent and polyvalent anions, which, under

appropriate conditions, can be produced within the The carbonate on the resin from Eq 6 can be resin.2aJ2 The suggested net reaction for the uptake of

directly incorporated into an exchangeable and highly cationic U022+ by chloride form anion resin is preferred uranyl tricarbonate anion starting with the neutral uranyl carbonate complex, according to Eq 8. 2 RCl + 2 U022+ + 4 HCO,- = R,UO,(CO,),

+ H2C03 taq) 2 R2CO3 + UO,(CO3)-R,UO,(CO3)3 (8) +2C1-+2H+

t U02C03 (9) Eqs 338 explain how an apparently neutral

species-UO,CO,a-can be taken up by anion resins In Eq 9, the divalent-selective resin sites in con- that strongly prefer the charged uranyl carbonate junction with HCO,- ions in the water promote the complexes or can convert bicarbonate to carbonate formation and uptake of divaIent UO2(CO3)2- from with the expulsion of a proton. (Essentially all strong cationic U022+, with the corresponding production of base resins are capable of performing this latter H,CO, and HCl. In contrast to Eq 5, Eq 9 produces a process20) As seen in Eqs 3-8, HCO,- drives the sys- neutral UO2CO3 molecule not retained by the resin. tern because it is the source of the COa2- ions that This helps to explain the 50 percent uranium leakage further complex the uranyl ion and make it progres- observed at pH 4.3 which was not observed at the sively more negatively charged. Table 321 illustrates higher, but still acidic, pH of 5.8. that sufficient bicarbonate was available irrthe Chim- The protons produced by Eq 9 will either add to ney Hill water to promote U02(C03)o uptake at pH 5.8. the H+ concentration or be consumed by HCO,- to Based on the waters total alkalinity of 150 mg/L as form H,CO, (aq). To convert the U022+ to a 50-50 CaCO,, its C,,,, was 3.00 mmol/L. Thus, at the nat- mixture of UO~CO~O-IJO~(CO~)~- would require 2

APRIL 1994 235

mmol HCO,-/mmol UO12+. In the Chimney Hill water this conversion would produce about 0.001 mmol/L H+, which might easily be consumed by what remained available of the original 0.026 mmol/L HCO,-. In summary, the carbonate species fraction changes and the pH changes produced by Eqs 5 and 9 would not be measurable in the bicarbonate- buffered Chimney Hill water because of the stoi- chiometrically small amount of uranium present.

Figure 7 shows the uranium-dumping effect caused by reducing the feedwater pH to 4.4 after par- tially exhausting the anion resin at pH 8.0. This col- umn was first operated at pH 7.6-8.2 for 915 h with a total of 41,100 BVs of throughput, during which the maximum uranium leakage was ~0.1 pg/L. Then, the pH of the feedwater was reduced and controlled at 4.2-4.5 by adding 0.2 N HCl. Serious uranium leakage (60 pg/L) was observed immediately after the pH was reduced. The uranium concentration in the effluent rose sharply to 500 from 0.1 pg/L within 400 BVs after lowering the pH. At 1,600 BVs follow- ing the pH reduction, the uranium concentration reached 2,000 pg/L, i.e., 17 times the feed concen- tration. Clearly, lowering the pH caused uranium to be dumped from the partially exhausted SBA resin. The proposed mechanism of dumping is shown in Eq 10.

ZR,UO,(CO,), + 4H,CO, -UOz2+ + 2HCO,- + R,UO,(CO,), + 6 RHCO, (10)

In Eq 10, after partial exhaustion at pH 8.0, eight resin sites (2 x R4+) are occupied by two uranyl tri- carbonate complexes [2 x U02(C0,)a4-1. When the pH 4.4 feedwater containing H,CO, contacts the sites

occupied by the uranyl tricar- bonate complexes, U022+ is released to the aqueous phase, while the resin retains some uranium as the divalent dicarbonate complex-UO, (COs)22-. During this reaction, the pH increases slig,htly as the H,CO, is neutralized (converted to HCO,-) by the uranyl carbonate complexes. It is known that not all the uranium is eluted from the resin at pH 4.3 because of the results from run 3 3.1 (Figure 6) just discussed. During run 33.1, virgin resin achieved 50 percent uranium removal at pH 4.3.

In summary, the uptake of uranium from carbonate buffered solutions containing predominantly cationic and neutral uranium complexes at pH 4.2-6.0 can be explained

by ( 1) the very high affinity of strong base resins for polyvalent anions in general and uranyl carbonate complexes in particular, (2) the ability of strong base resins to produce divalent CO,2- from HCO,-, and (3) the relatively high buffer capacity of the carbonate species present. Resins containing uranyl carbonate complexes exhibit significant buffer capacity. Such uranium-loaded resins can release uranium into the aqueous phase when the feed pH is lowered signifi- cantly, i.e., to ~4.6 in these experiments.

Effects of uranium and sulfate concentrations on uranium breakthrough. Uranium feed conccn- tration was expected to have a big influence on col- umn capacity; however, because of the extremely long runs and the potential regulatory problems asso- ciated with uranium spiking of the raw water, it was not practical to vary the uranium concentration in the Chimney Hill water to determine its influence on bed volumes to uranium breakthrough. Thus, computer predictions of uranium breakthrough were made using the Equilibrium Multicomponent Chro- matography Program with Constant Separation Fac- tors (EMCT-CSF).23 This program is a user-friendly implementation of the multicomponent chromatog- raphy theory of Helfferich and I

203,000 BVs. Decreasing the uranium concentration in groundwaters containing the usual carbonate and to 20 pg/L would increase the run length to 815,000 bicarbonate concentrations. This is the justification BVs. This high sensitivity of run length to uranium for excluding uranyl sulfate complexes from the concentration is due to the fact that uranium is an computer predictions. exceptionally highly preferred species and occupies a Regeneration. Effect of NuCZ concentration. The significant fraction of the resin sites at exhaustion. results of the pure NaCl regenerations on columns

The EMCT-CSF program was also used to pre- 11 and 27 are summarized in Figures 9-12. NaCl diet the influence of sulfate on uranium run length concentrations ranging from 0.8 to 4.0 N were applied at variable uranium concen- trations. In this simulation, sulfate replaced chloride in the Chimney Hill water, which is naturally almost free of sul- bough not well documented, several

rail community water supplies have uranium concentrations in the high micrograms-per-litre range-even up to the milligrams-pelclitre range.

fate. The line labeled Sulfate water in Figure 8 shows that replacing chloride with sulfate greatly affects the SBA resins capacity for uranium removal. At the natural uranium con-

sulfate concentration increased centration of 122 ug/L, if the

from 0 to 1.32 meq/L (64 mg/L), run length drops from the experimental 340,000 BVs to a predicted 135,000 BVs, a decrease of 60.3 percent. This decrease at higher sulfate concentration is attrib- uted to the fact that compared with chloride, sulfate ion has a strong affinity for the SBA resin sites. Therefore, water containing significant concentra- tions of sulfate will produce significantly shorter run lengths than low-sulfate waters. As shown in Table 1, the stability constants of the uranium sul- fate complexes [U0,(S0,),4- and U0,(S0,),2-] are much lower than those of uranium carbonate com- plexes. Thus, it is expected that uranium sulfate complexes will not occur in any significant degree

a regeneration level of 6.0 eq NaCl/eq resin (2 1.6 lb NaCl/cu ft resin), the recovery effi- cienciesfor4.0, 3.0,2.0, 1.33, 1.0, and 0.8 N NaCl regener- ant were 91, 86, 75, 67, 54, and 47 percent, respectively, as shown in Figure 10. The

to columns partially exhausted to 30,000 or 41,000

uranium recovery did not appear to level off at 4.0 N,

BVs to establish the effect of regenerant concentration

which suggests that even higher NaCl concentration

on uranium recovery efficiency. Although the NaCl

might more closely approach 100 percent uranium recov-

concentration varied, the total amount of NaCl applied

ery. Thus, the use of saturated NaCl (6.15 N at 2OoC) to

was maintained constant at 4.0, 6.0, or 10 eq NaCl/eq

regenerate uranium-spent columns seems appropriate.

resin (14.4, 2 1.6, or 36 lb NaCl/cu ft resin) during three sets of experiments.

Figures 9-12 demonstrate that uranium recovery efficiency strongly depends on the regenerant NaCl concentration. Increasing NaCl concentration resulted in increased uranium recovery at a fixed regeneration level. As shown in Figure 9, eluted uranium exhib- ited a narrow peak at high NaCl concentration and a relatively broader peak at low NaCl concentration. At

,)I&& 8 Bed volume throughput calculated for a resin column before uranium

1 breakthrough at different concentrations of uranium, chloride, and sulfate

Effect of NaCZ regenera- tion level. The results of using two NaCl regeneration lev- els (4.0 and 6.0 eq Cl-/eq resin) are also shown in Fig- ure 10, which indicates that uranium recovery depends less on regeneration level

APRIL 1994 237

FlOURa-4) Uranium elution by OS, 1.0, 2.0, or 3.0 N NaCl during first cocurrent regeneration of a resin bed erh=l=*ad *a 30,000 BVs; regeneration level-10 eq Cl-/eq resin

than on the NaCl concentration. Nevertheless, increasing the regeneration level from 4.0 to 6.0 eq Cl-/eq resin did tend to improve uranium recovery, with bigger improvements noted at 1.3 N and higher NaCl concentrations.

In Figure 9, it is clear that uranium was mostly des- orbed in the first few bed volumes during regenera- tion, especially at higher NaCl concentrations. For example, examination of the elution curve during 2 N NaCl regeneration shows that the area under the portion of the curve from 2 to 5 BVs is

at high pH values after the addi- tion of strong base. The formed precipitates were then retained in the resin pores or stuck to the resin structure and were not eluted by the regenerant, which caused the substantially lower uranium concentration in the regeneration elute. As reported, in uranium mining practice, strong base is added to the regenerant elute to pre- cipitate uranium. 17,26-29 Dur- ing regeneration by NaCI, the uranyl tricarbonate complex is first replaced from resin sites by chloride. Upon the addition of strong base, the regenerated uranyl tricarbonate ions react with NaOH and produce sodium pyrouranate or ura- nium yellow, Na2U20,,26J7 which, like all the other uranates, is insoluble. The sug- gested reactions during this process are as follows:

R,UO,(CO,), + 4 NaCl-4 RCl + uo2(co3)34- +4Na+ (11)

2 U02(C0,),4- + 6 OH- + 2 Na+ = Na,U,O, (s) + 6 C0,2- + 3 H,O (12)

In Eq 11 uranyl tricarbonate is eluted from the resin by excess chloride. Then, in Eq 12 the eluted tri- carbonate combines with hydroxide to form the insol- uble pyrouranate. Adding Eqs 11 and 12 yields:

2 R,UO,(CO,), + 8 NaCl+ 6 NaOH= 8 RCI + Na2U207 (~1 + 6 C032- + 3 H,O + 12 Na+ (13)

Along with these reactions, the production of ura- nium oxides is also possible. 17,28-30 According to this mechanism, uranyl tricarbonate generated during regeneration may react with NaOH and produce insol- uble uranium oxides in the presence of strong base. The suggested reaction is

U02(C03)34- + 2 NaOH = UO, (s) + 3 CO,2- + 2 Na+ + H,O (14)

For Eq 14, there may be some uranyl hydroxide complexes formed as intermediate products, which hydrolyze to UO, at very high pH values.izJs-30 Adding Eq 11, the elution of uranium tricarbonate, to Eq 14 gives the net reaction

R,UO,(CO,), + 4 NaCl + 2 NaOH= 4 RCl t UO, (s) + H,O t 3 co,2- t 6 Na+ (15)

When reducing agents are present, the production of U,Os, a mixture of UO, and UO,, is possible.

In summary, Eqs 11-15 explain how insoluble uranium compounds including sodium pyrouranate or uranium oxides are produced from uranyl tri- carbonate complex during regeneration by NaCl with the addition of NaOH. Similar reactions can be written for uranyl dicarbonate complex, u2(co3)2 2-, when it is released from the resin sites. It appears that the resulting uranium precipi- tates are mostly retained in the resin, which sub- stantially reduces the uranium recovery during regeneration.

In contrast to the negative effect of NaOH on regeneration by NaCl, pure HCl exhibited a slightly higher uranium recovery than pure NaCl at 1 N con- centration. This is probably because under acidic con- ditions the uranyl tricarbonate complex is converted to cationic U022+, which is easily eluted from the positively charged anion resin. These results agree with the prediction that the addition of HCI to NaCl regenerant will enhance uranium recovery.al How- ever, it is questionable whether the small increase in uranium recovery by HCI is worth the increased chemical and corrosion prevention costs.

Effect of percentage resin exhaustion. Because resin beds are seldom if ever operated to 300,000 BVs, SBA resins were exhausted to 500, 31,000, or 41,000 BVs and then regenerated with 6 eq 2.0 N NaCl to study the effects of the degree of uranium exhaustion on the regeneration efficiency. Compared with run E4.1 which was terminated at 302,000 BVs,

APRIL 1994 239

FMMJRE f2 Influence of regenerant stoichiometry and NaOH addition on uranium recovery during regeneration with 2.0 PI NaCl of resin exhausted to

runs 11 and 27, which were operated only to 3 1,000 and 41,000 BVs, respectively, were far from com- plete uranium exhaustion. When the beds were regenerated with 6 eq of 2.0 N NaCl, uranium recov- ery efficiency was constant in the range from 75 to 78 percent, irrespective of the run length of 50041,000 BVs. The uranium leakage after cocurrent resin regen- eration was insignificant and similar to that of the virgin column for all three runs.

Summary and conclusions This uranium removal study was conducted on a

natural, well-buffered, and almost sulfate-free groundwater containing 11 O-l 2 5 pg/L uranium and 25 pCi/L radium. The macroporous SBA resin used exhibited enormous capacity for removing uranium. A bed of this resin was operated continuously for 478 days at pH 7.6-8.2 for a total throughput of 302,000 BVs, at which time its effluent uranium concentration was still

the members of Chimney Hill MUD Water Board, especially Jay Sinzei, president, for their cooperation and support.

References 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

US Environmental Protection Agency, National Primary Drinking Water Regulations; Radionu- elides; Advanced Notice of Proposed Rulemaking. Fed. Reg., 56:138 (40) CFR Parts 141 and 142 (1991). SORG, T.J. Methods for Removing Uranium From Drinking Water. Jour. AWWA, 80:7:105 (July 1988). CLIFFORD, D.A. 6 ZHANG, Z. Modifying Ion Exchange for Combined Removal of Uranium and Radium. Jour. AWWA, 86:4:214 (Apr. 1994). LANGMUIR, D. Uranium Solution-Mineral Equi- libria at Low Temperatures With Applications to Sedimentary Ore Deposits. Geochemica et Cos- mochimica ( 1978). MO, T.J. Chemistry of Uranium in Aqueous Envi- ronments. USEPA Ofce. of Radiation Programs, Criteria 6 Standards Div., Radioactive Waste Standard Br. ( 1980). BRUSILOVSKII, S.A. Tr. IGEM Akad. Nauk SSSR (1960). KOMAR, N.P. Osnovy Kachestvennogo Khimicheskogo Analiza, 1 (Fundamentals of Qualitative Chemical Analysis, I). Kharkov (1955). SCANLAN, J.P. Equilibria in Uranyl Carbonate Sys- tems-11, The Overall Stability Constant of U0,(C03)22- and the Third Formation Constant of U02(C0,),4-. Jour. Inorg. Nuclear Chem., 39:635 (1977). NELSON, F. 6 KRAUS, K.A. Chemistry of Aqueous Uranium (v) Solutions. III. Jour. Amer. Chem., 73:5:2157 (1951). YATSTMIRSKII, K.B. 6 VASILYEV, V.P. Konseanty Neotoikosti Kompleksnykh Soyedinenii (Insta- bility Constants of Complex Compounds). Izd. Akad. Nauk. (1959). BROWNE, E.; FIRESTONE, R.B.; &- SHIRLEY, V.S. Table of Radioactive Isotopes. John Wiley & Sons, New York (1986). LONGTIN, J.P. Occurrence of Radon, Radium, and Uranium in Groundwater. Jour. AWWA, 80:7:84 (July 1988). LOWRY, J.D. S LOWRY, S.B. Radionuclides in Drinking Water. Jour. AWWA, 80:7:51 (July 1988). BONDIETTI, E.A.; LEE, S.Y.; &- HALL, S.K. Meth- ods of Removing Uranium From Drinking Water: II-Present Municipal Treatment and Potential Removal Methods. EPA 570/9-82- 003 (1982). LASSOVSZKY, P. 6 HATHAWAY, S.W. Treatment Tech- nologies to Remove Radionuclides From Drink- ing Water. Presented at the USEPA Natl. Work- shop on Radioactivity in Drinking Water, May 23-26, 1983. HATHAWAY, S.W. Uranium Removal Processes. Proc. 1983 AWWA Ann. Conf., Las Vegas, Nev.

17. MILLER, W.S. 6 MINDLER, A.B. Ion Exchange Sep- aration of Metal Ions From Water and Waste Waters. Recent Development in Separation Science. CRC Press, Cleveland, Ohio (1977).

18. JELINEK, R.T. S SORG, T.J. Operating a Small Full- Scale Ion Exchange System for Uranium Removal. Jour. AWWA, 80:7:79 (July 1988).

19. Standard Methods for the Examination of Water and Wastewater. APHA, AWWA, and WPCF, Wash- ington, D.C. (16th ed., 1985).

20. HORNG, L.L. Reaction Mechanisms and Chro- matographic Behavior of Polyprotic Acid Anions in Multicomponent Ion Exchange. Doctoral dis- sertation, Univ. of Houston, Texas ( 1983).

2 1. SNOEYINK, V.L. 6 JENKINS, D. Water Chemistry. John Wiley S Sons, New York ( 1980).

22. SENGUPTA, A.K. Chromate Ion-exchange Study for Cooling Water. Doctoral dissertation, Univ. of Houston, Texas (1984).

23. CLIFFORD, D.A. 6 MAJANO, R.E. Computer Pre- diction of Ion Exchange. Jour. AWWA, 85:4:20 (Apr. 1993).

24. HELFFERICH, F. 6 KLEIN, G. Multicomponent Chro- matography. Marcel Dekker, New York (1970).

25. CLIFFORD, D.A. Multicomponent Ion-Exchange Calculations for Selected Ion Separations. Jour. Indus. Engrg. Chem. Fundamentals, 2 1 (1982).

26. BAILAR, J.C. JR. ET AL. Comprehensive Inorganic Chemistry. Pergamon Press, London (1973).

27. THORNE, I?C.L. 6 ROBERTS, E.R. Inorganic Chemistry. Interscience Publishers, New York (6th ed., 1954).

28. SMITH, S.E. S GARRE?T, K.E. Some Recent Devel- opments in the Extraction of Uranium From Its Ores. Chem. Engrg. Jour. (Dec. 1972).

29. The Uranium Institute. Ufanium and Nuclear Energy. Proc. 4th Intl. Symp., London, Sept. 10-12, 1979.

30. HANSON, S.W.; W.JILSON, D.B.; 6 GUNA.JI, N.N. Removal of Uranium From Drinking Water by Ion Exchange and Chemical Clarification. EPA Contractor CR-810453-01-0, EPA 6OO/S2- 87/076. Cincinnati, Ohio ( 1987).

31. MINDLER, A.B. & TERMINJ, J.P.K. The Vital Role of Ion Exchange in Uranium Production. Engrg. &Mining Jour., 157:9 (Sept. 1956).

About the authors: Zhihe Zhang is a postdoctoral research associate in the Department of Environmental Engineer, University of Houston, 4800 Calhoun St., Houston, TX 77204- 4791. A graduate of the Chinese Uni- versity of Science and Technology, Peking, with a bachelor k degree and

of the University of Houston with a doctoral degree, Zhang has more than 15 years of experience in the water and wastewater treatment, power, and petroleum-chemical industries. Zhang is a member of AWWA and ACS, and his work has been published previously by Journal AWWA. Dennis Clifford is professor of environmental engineering at the University of Houston.

APRIL1994 241