Removal of Lead and Cadmium from Aqueous Waste Streams Using Granular Activated Carbon

(GAC) Columns

Brian E. Reed and Selvam Arunachalam Department of Civil and Environmental Engineering,

West Virginia University, Morgantown, WV 26506

and

Bob Thomas Norit Americas Inc., 1050 Crown Pointe Parkway, Atlanta, GA 30338

The use of granular activated carbon (GAC) columns to treat metal-bearing wastewaters was investigated. Synthetic wastewaters containing Pb and Cd ( I0 or 50 mg/L) , acetic acid (0.001 N ) or EDTA (1:O.I or 1:1 Me:EDTA molar ratios)

were studied. For metal-only and metal-acetic acid experiments, significant quantities (as high as 325 bed volumes ( B V ) ) of wastewater were treated prior to breakthrough (Ce=0.03 Co). X / M values were as high as about 30 mg Pb/g carbon. For EDTA experiments, C, was always>0.03 C,. The amount of metal

not removed corresponded to the amount that was complexed by EDTA. Column pH is the critical parameter influencing column performance. The increase in

effluent metal concentration corresponded with the decrease in column pH. GA C columns were successfully regenerated using a 1 L ( = 8 B V ) 0. I N HNO, rinse followed by a I L 0.1 N NaOH rinse. Column performance was not adversely

affected by regeneration. When the regeneration step was used on virgin carbon, a dramatic improvement in column performance was observed and was attributed to the increase in carbon surface p H (pH= II) and the deposition of OH- in the pore liquid. Possible removal mechanism are precipitation on the carbon surface

and in the pore liquid, and adsorption.

INTRODUCTION

The presence of heavy metals in the environment is of major concern because of their toxicity and threat to human life and the environment. In 1983, an estimated 7.9 billion gallons of heavy metal-bearing wastewaters were generated in the U.S. [I]. Lead, cadmium, and mercury are examples of heavy metals that have been classified as priority pollutants by the U.S. Environmental Protection Agency. Anthropogenic sources of heavy metals include wastes from the electroplating and metal finishing industries, metallurgical industry, tannery opera- tions, chemical manufacturing, mine drainage, battery man- ufacturing, leachates from landfills, and contaminated groundwater from hazardous waste sites.

For wastes with high metal concentrations, precipitation processes (e.g., hydroxide, sulfide) are the most economical. However, many metal-bearing wastes contain substances, such as complexing agents, that decrease the effectiveness of pre- cipitation processes leading to relatively high metal concen- trations in the effluent. In addition, the current regulatory trend is for heavy metal discharge limits approaching those of

drinking water standards. Thus, additional treatment proc- esses, downline from the precipitation process, may be required to “polish” the effluent prior to discharge. These tertiary processes may also be the primary metal removal process for waste streams having low concentrations of metals. Examples of such processes are ion exchange, reverse osmosis, and ad- sorption. Ion exchange and reverse osmosis, while effective in producing an effluent low in metals, have high operation and maintenance costs and are subject to fouling. Adsorption by activated carbon is an established treatment method for organic contaminants. Although the ability of activated carbons to remove heavy metals has been established by numerous re- searchers [2-6], they have not been used in an actual treatment setting for inorganic adsorbates. The majority of these studies were conducted in the batch mode. If activated carbon is to be considered as a viable method for heavy metal removal, additional information on its efficiency in the column mode must be gathered.

The goal of this research was to demonstrate the efficacy of using granular activated carbon (GAC) columns to treat heavy-metal bearing waste streams. Lead and cadmium were

60 February, 1994 Environmental Progress (Vol. 13, No. 1)

Table 1 Characteristics of Hydrodarco 4000 Surface Area (N2 BET Method), m2/g Slurry pH Ash Content, % Molasses Number Iodine Number Mean Pore Radius, angstroms Total Pore Volume, mL/g Apparent Density, g/mL Particle Density Wetted, g/mL Mean Particle Diameter, mm Effective Size Uniformity Coefficient Particle Size Distribution, U.S. Sieves + 12

12x 16 16 x 20 20 x 30 30 x 40 40 x 50

- 50

625 5.2 23 812 647 29

1.04 0.37 1.4 1.09 0.74 1.44

Percent 5.86 19.12 34.87 38.75 1.05 0.13 0.23

chosen as the study heavy metals because of their presence on the USEPA priority pollutant list. Darco HD4000 (American Norit Company, Inc.) was selected as the study activated car- bon because the authors have investigated this carbon in earlier work. Specific objectives of the research were to: 1) conduct GAC column studies using synthetic waste streams containing lead and cadmium, organic compounds (EDTA and acetic acid), and different influent pH values, and 2) study the effect of regeneration on GAC column performance.

METHODOLOGY

Lead and cadmium concentrations of 10 and 50 mg/L, were used for each of three synthetic wastewaters investigated. The characteristics of the column influents were as foIlows: 1) lead or cadmium only at pHs of 4 (Pb only) and 5.4, 2)10-3 M acetic acid at pH = 4.7, and 3) EDTA at Me:EDTA molar ratios of 1:O.l and 1:1, pH = 5.4. Wastewaters 2 and 3 were used to simulate wastes containing weak organic acids and strong complexing agents, respectively. Lead and cadmium were analyzed by flame atomic absorption spectrophotometry using a Perkin-Elmer 2380 atomic absorption spectropho- tometer. Samples were stored in polyethylene bottles and ac- idified using concentrated HN03 immediately following sam- pling. The pH was measured using a Fisher Scientific Accumet mini pH meter (Model 640 A). All stock solutions were for- mulated using reagent grade chemicals. Experiments were con- ducted at an ionic strength of 0.01 (added as NaNO,) and a temperature of about 22°C. Hydrodarco 4000, the study ac- tivated carbon, was supplied by Norit Americas Inc. Selected characteristics of Hydrodarco 4000 are presented in Table 1.

Approximately 50 g (dry) of the carbon was weighed, sieved through a U.S. No. 50 sieve, and washed with distilled water to remove fines. The carbon was placed in 15 inch long, 1 .I25 inch inner diameter acrylic columns such that the formation of air voids was minimized. Rounded stones and glass wool were used as the column support media. The length of the adsorptive bed was approximately 7.5 inches and the bed vol- ume was 0.12 L. A hydraulic flow rate of 40 mL/min (6.24 cm3/cm2, 1.5 gal/ft*.min) was maintained using a Cole-Parmer peristaltic pump with Masterflex speed controller. Columns were operated in the up flow mode. Effluent samples were

InfIuent p H : 5.47 D a r c o H D 4 0 0 0 : 50 g

0.8 - 0 0

U 2

0.4

0.2

v Run 2 0 Run 3 A Run 4

0.0 0 100 200 300 400

B e d Volumea

1 2 , I I I I I

- 0 100 zoo 900 400

B e d Volumea

FIGURE 1. Breakthrough and effluent pH curves for 10 mglL Pb and pH = 5.4.

collected either every 30 or 60 minutes, measured for pH, and acidified. During the course of the column cycle (opera- tion + regeneration) the volume and metal concentration of the influent, effluent (including samples), and regenerants were measured and used in mass balance calculations. Mass balance results were used to calculate surface concentrations (X/M), mass of metal removed, and desorption efficiencies. Following each column run, carbon was regenerated using a procedure consisting of 0.1 N HN03 and 0.1 N NaOH rinses.

Approximately 1 liter of 0.1 N HNO, was pumped through the column at a flow rate of 10 mL/min. Samples of the acid rinse were taken periodically and measured for pH and metal content. This procedure was repeated using 1 liter of 0.1 N NaOH. The 0.1 N NaOH remaining in the column after the base rinse was allowed to contact the carbon for approximately five days. The five day contact time was chosen strictly on logistics (i.e., it was the minimum amount of time before the new column run could begin). Following regeneration, the column cycle was repeated. Depending on the experiment, the column cycle was repeated either three or four times. Metal removal dramatically increased following the first regeneration step. Thus, in later experiments, the regeneration procedure was used as a carbon pretreatment step for virgin carbon (i.e., before the first column run). Desorption efficiency was de- termined using mass balance calculations.

RESULTS AND DISCUSSION

For ease of presentation, results from each of the synthetic wastewaters are presented separately.

Environmental Progress (Vol. 13, No. 1) February, 1994 61

1.0

0.8

50 mg/L Cadmium Influent pH: 5.4

0.6 Carbon: 50 g EBV: 0.12 L

0 Run 1 v Run 2 o Run 3

0 50 100 150 200 250 300 350 400 450

Bed Volumes

0 50 100 150 200 250 300 350 400 450

Bed Volumes

FIGURE 2. Breakthrough and effluent pH curves for 5 mglL Cd and pH = 5.4.

Metal-Only

Normalized effluent lead concentration (C,/C,) versus num- ber of bed volumes (BV) treated for 10 mg/L, pH = 5.47 are presented in Figure 1. These curves will be referred to as break- through curves for the remainder of this article. Breakthrough was defined at C, = 0.03 C,. Breakthrough occurred at about 50 BV for Run 1. Lead removal increased by approximately 600 percent following the 0.1 N HN03 - 0.1 NaOH N regen- eration step. Based on the lead-only results, the regeneration procedure was used as a pretreatment step for virgin carbon for the remainder of the column experiments. Also presented in Figure 1 is the effluent pH versus BV treated. Effluent pH for Run 1 decreased from about 7 at the beginning of column operation to 5.4 at the end. Initial effluent pH for Runs 2 through 4 were greater than 11 and decreased to about 8 at breakthrough. Breakthrough and effluent pH curves for 50 mg/L Cd, pH = 5.4 are presented in Figure 2. While the shape of the breakthrough and pH curves are similar, much less cadmium was removed compared to lead. In Table 2, a sum- mary of column performance for the metal-only column studies is presented.

It is hypothesized that lead was removed in greater quan- tities, compared to cadmium, because of its solution chemistry. Lead begins to form hydroxide species at pH = 5 and forms Pb(OH)* (s) at about pH = 5.5 to 6 while, cadmium begins to form hydroxide species at p H = 8 and forms Cd(OH), (s) at pH= 8.5 to 9. The lower the pH at which metal hydroxide species form, the better the removal. Thus, it was expected that lead would be better removed compared to cadmium.

Table 2 Summary of Column Performance for Metal- Onlv ExDeriments

Influent BV Treated 10 mg/L Pb - pH = 4 Run 1 40 Run 2 250 Run 3 240 Run 4 235

10 mg/L Pb - pH = 5.4 Run 1 50 Run 2 325 Run 3 315 Run 4 300

Run 1 20 Run 2 110 Run 3 110 Run 4 110

Run 1 25 Run 2 125 Run 3 120 Run 4 120

10 mg/L Cd - pH = 5.4 Run 1 80 Run 2 80 Run 3 80

Run 1 40 Run 2 40 Run 3 40

50 mg/L Pb - pH = 4

50 mg/L Pb - pH = 5.4

50 mg/L Cd - pH = 5.4

Me Removed, X/M, mg mg Me/g carbon

48 0.97 306 6.11 272 5.43 249 5.00

23 0.46 35 1 7.01 359 7.18 368 7.36

282 5.64 1280 25.6 1320 26.4 1370 27.4

341 6.81 1340 26.8 1440 28.7 1550 31.0

88 1.76 91 1.83

101 2.00

290 5.76 240 4.76 230 4.63

GAC columns were successfully regenerated using a rela- tively simple procedure consisting of 1 liter (= 8 BV) rinses of 0.1 N HN03 and NaOH. Column performance was not ad- versely affected by the regeneration procedure. The dramatic improvement in column performance after the first regener- ation is attributed to an increase in OH available for surface and pore liquid precipitation as well as an increase in the number of surface sites available for adsorption. The regen- eration procedure as a pretreatment step for virgin carbon is recommended.

Lead-Acetic Acid

Acetic acid was used to simulate a moderately acidic waste- water such as a landfill leachate. Breakthrough and effluent pH curves for 0.001 M acetic acid and 10 mg/L Pb and Cd are presented in Figures 3 and 4, respectively. A summary of column performance for acetic acid experiments is presented in Table 3.

Approximately 120 BV of 10 mg/L Pb and 70 BV of 50 mg/L Pb were treated at breakthrough for each of the three column runs. The presence of acetic acid caused a deterioration of the column performance compared to the lead-only exper- iments. Because of differences in influent pH, it is not possible to directly compare the two systems. However, if results for the lead-only system are interpolated to pH = 4.7, the presence of acetic acid decreased the BV treated by about two-thirds for 10 mg/L and one-third for 50 mg/L. As in the lead-only column studies, the deterioration in column performance co- incided with the rapid drop in column pH. The increase in waste acidity accelerated this phenomenon and caused break- through to occur earlier than in the lead-only experiments. At 10 mg/L Pb, the run number did not appear to effect column performance. For 50 mg/L Pb, the BV treated, mass of lead

62 February, 1994 Environmental Progress (Vol. 13, No. 1)

2.2 , I I I I

0.6

0.4

0.2

2.0

1.0

1.6

1.4

Q 1.2

u 1.0

0.8

0.6

0.4

0.2

0.0

U

2

-

-

-

Influent p H : 4 . 7 0.001 M CH,COOH Darco H D 4 0 0 0 : 50 g BV: 0.12 L

0 100 200 so0 400

Bed Volumes

g 10 4

* 2 6

A 0 100 200 a00 400

Bed Volumer

FIGURE 3. Breakthrough and effluent pH curves for 10 mglL Pb, 0.001 M acetic acid, and pH =4.7.

removed, and lead surface concentration decreased slightly after Run 1 but, were constant for Runs 2 and 3. Exhaustion of the columns (defined as the point where the breakthrough curve first intersects the CJC, = 1 line) occurred between 150 and 175 BV for 10 mg/L and between 100 and 120 BV for 50 mg/L Pb. At exhaustion, effluent and influent pH were equal

The presence of acetic acid did not appear to significantly affect cadmium removal compared to the cadmium-only sys-

(pH = 4.7).

Y v B 10 mg/L Cadmium Influent pH: 4.7 0.001 M CH,COOH Carbon: 50 g BV: 0.12 L

v Run 2 0 Run 3

0 50 100 150 200 250 300 350 400 450

Bed Volumes

FIGURE 4. Breakthrough and effluent pH curves for 10 mglL Cd, 0.001 M acetic acid, and pH = 4.7.

tems. It is unclear why the increase acidity (via acetic acid) did not decrease removal. Possibly, the solution chemistry of cad- mium played a role. In the future, additional research will be conducted to shed light on this phenomenon.

For both lead and cadmium, desorption occurred as the column pH decreased. For metals present in cationic form,

Table 3 Summary of Column Performance for Metal-Acetic Acid Experiments Influent BV Treated' Me Removed', mg X/M', mg Me/g carbon 10 mg/L Pb - pH = 4.7 Run 1 120; 155 Run 2 100; 145 Run 3 120; 170

Run 1 75; 120 Run 2 70; 110 Run 3 70; 110

10 mg/L Cd-pH=4.7 Run 1 80; 120 Run 2 75; 110 Run 3 70; 120

Run 1 40; 95 Run 2 40; 90 Run 3 40; 115

50 mg/L Pb - pH = 4.7

50 mg/L Cd - pH = 4.7

' First number at breakthrough; second number at exhaustion.

127; 162 129; 153 149; 179

483; 626 421; 522 416; 523

87; 110 80; 100 78; 106

225; 350 175; 320 240; 375

2.55; 3.23 2.57; 3.06 2.97; 3.58

9.66; 12.5 8.42; 10.4 8.32; 10.5

1.73; 2.20 1.59; 2.00 1.56; 2.13

4.50; 7.00 3.50; 6.43 4.77; 7.53

Environmental Progress (Vol. 13, No. 1) February, 1994 63

10 mg/L Lead Influent pH: 5.47 Pb:EDTA 1:O.l Darco HD4000: 50 g BV: 0.12 L

0 V \

V

0.2 1 v Run 2 0 Run 3

0.0 I I I I I I 0 100 200 so0 400

Bed Volumes

i 4

0 100 200 300 400

Bed Volumes

FIGURE 5. Breakthrough and effluent pH curves for 10 mglL Pb, Pb:EDTA ratio of l:O.l, and pH =4.7.

adsorption decreases with a decrease in pH. Thus, as the col- umn pH decreased towards the influent pH (pH = 4.7), metal desorbed from the carbon. Cadmium desorbs at a higher pH compared with lead because Cd2' is formed at a higher pH (as was discussed earlier, Me2+ is less adsorbable than Me(OH)+) and, Cd(OH);! (s) dissolves at a higher pH compared to Pb(OH);! (s). It is apparent that solution chemistry plays a key role in metal removal by activated carbon.

Lead-EDTA

Breakthrough and effluent pH curves for 10 mg/L Pb at a Pb:EDTA molar ratio of 1:O.l are presented in Figure 5 . For all runs, C, was greater than 0.03C0 at the beginning of the column run. Thus, based on the definition of breakthrough, zero BV were treated. According to thermodynamic calcula- tions, 10 percent of the lead forms complexes with EDTA at this pb:EDTA molar ratio. For the entire column run about

10 percent of the lead was not removed. Thus, it is hypothesized that the Pb-EDTA complex was not adsorbed by the carbon. Similar results were observed for the Pb:EDTA molar ratio of 1 : 1 and Cd:EDTA molar ratios of 1 :O. 1 and 1 : 1.

CONCLUSIONS

Significant quantities of metal-bearing wastewaters were treated using GAC columns. Column pH and influent char- acteristics appear to be the critical parameters influencing GAC column performance. The sudden increase in the column ef- fluent lead concentration corresponded noticeably with a de- crease in the effluent pH. Acetic acid at 0.001 M decreased column performance compared with the lead-only experiments but large amounts of lead were still removed from solution. It is hypothesized that acetic acid reacted with the carbon associated OH- (on the surface and in the pore liquid), which produced a more rapid decrease in column pH and a subsequent decrease in lead removal. The presence of EDTA adversely affected column performance most likely because of solution complexation. Wastewaters containing strong complexing agents will either have to undergo pretreatment prior to en- tering the GAC column or an activated carbon that adsorbs the organic-metal complexes will have to be identified.

GAC columns were successfully regenerated using a rela- tively simple procedure consisting of 1 liter (= 8 BV) rinses of 0.1 N HNO, and NaOH. Column performance was not ad- versely affected by the regeneration procedure. When the re- generation step was used on virgin carbon, a dramatic improvement in column performance was observed. This was attributed to an increase in OH- available for surface and pore liquid precipitation as well as an increase in the number of surface sites available for adsorption. The use of the regen- eration procedure as a pretreatment step for virgin carbon is recommended. Given the encouraging performance of the GAC columns in treating several lead wastewaters and the relative ease of column regeneration, the use of GAC columns to treat metal-bearing waste streams should be considered.

LITERATURE CITED

1. Grosse, D. W., JournalAir Polhtion ControlAssociation, 36, p. 683, (1986).

2. Reed, B. E., and Matsumoto, M. R., Journal of Envi- ronmental Engineering Division, ASCE, 119(2), p. 332, (1993).

3. Reed, B. E., and Nonavinakere, S. K., Separation Science and Technology, 27(14), p. 1985, (1992).

4. Corapcioglu, M. O., and Huang, C. P., Water Research, 9(9), p. 1031, (1987).

5 . Netzer, A., and Hughes, D. E., Water Research, 18(8), p. 927, (1984).

6 . Huang, C. P., and Ostovic, F. B., Journal of Environ- mentalEngineering Division, ASCE, 104(5), p. 863, (1978).

64 February, 1994 Environmental Progress (Vol. 13, No. 1)