Removal of cadmium, zinc, and lead from wastewater using recycled shot-blast fines

Removal of Cadmium, Zinc, and Lead from Wastewater Using Recycled Shot-Blast Fines by Edward H. Smith, Environmental Engineering Program, Mechanical Engineering Department, Southern Methodist University, Dallas ncreasingly strict discharge limits on heavy metals have resulted in burgeoning compliance costs for the metal-finishing industry, motivating the search for economically attractive treatment technologies for metal removal. Sorption processes are promising in this regard, as opposed to more conventional chemical precipitation, in that they achieve higher level removals over a wider range of solution conditions and reduce the quantity of sludges or solid residuals that need to be disposed. One major challenge to the use of sorption processes is the relatively high cost of commercial sorbents such as activated carbon and ion exchange resins. Solid residuals from metal-related production processes may actually contribute to addressing these concerns. Waste shot-blast fines, derived from surface-finishing operations in the manufacturing of cast-iron components, exhibit considerable potential as a low-cost adsorbent for removal of heavy metals in aqueous waste streams. Depending on the size of the plant, several cubic yards of the residual iron-beating material may be generated daily. The material tests nonhazardous according to toxicity characteristic leaching procedure (TCLP) analysis, but is typically sent to a solid waste landfill for disposal, incurring associated transport and disposal costs. Preliminary inspection of a sample of the material, however, reveals that it possesses a significant iron component in addition to other oxide surfaces that are effective sorption sites for metals in aqueous systems. Iron-containing materials have been used widely in water and industrial wastcwater treatment for removal of heavy metal compoundsJ '2 Coagulant produced from electrolysis of iron has been demonstrated to be effective for reducing high concentrations of heavy metals from elcctroplating wastewaters and contaminated ground waters. 3"4

I

METAL FINISHING • NOVEMBER 1995

Amorphous iron oxide, or ferrihydrite, is a common surface coating of subsoil particles and has been shown to have a high capacity for hexavalent chromium as ionic chromate. 5 Other iron-containing minerals and iron hydroxide hybrids have demonstrated comparable adsorption capacity for heavy metals. 6.7 The ready availability of the residual iron material renders recycling and use of the material in treatment applications for metal-bearing wastewaters a potentially innovative and cost-effective venture. The objective of this study is to investigate experimentally the capacity and rate of removal of the heavy metals cadmium, zinc, and lead onto the iron sorbent in batch systems. Existing mathematical models for adsorption equilibria and kinetics are used to quantify the experimental data and serve as an aid in interpreting the results. EXPERIMENTAL Materials

A sample of waste iron fines was collected from an impact millroom wheelabrater at a cast-iron facility, which produces ductile iron pipes and fittings. Preparation of the recovered material for adsorption experiments involves sieving of various size fractions using U.S. standard sieves and storing in air-tight glass containers. Material for immediate use is dried to a constant weight and stored in a desiccator. Otherwise, the fines were used in adsorption experiments as is. The size fractions collected ranged from 20/30 mesh size to 200/325. Except where

noted, the 200/325 mesh size fraction was utilized in all experiments reported in this study. The surface area was evaluated by the three-point BET N 2 adsorption method using a Quantasorb surface analyzer. The surface area and several other relevant properties of the solid are presented in Table I. Cadmium, lead, and zinc were the metals examined in the study. Working solutions were prepared from concentrated stock solutions of Cd(NO3) 2, Zn(NO3) 2, and Pd(NO3) 2. Initial metal concentrations ranged from 0.5 to 25 ppm, and ionic background varied from 10 -3 to 10 -I M as NaCIO 4. pH adjustment was with either 1 N HCIO 4 or 1 N NaOH under a nitrogen, CO 2free atmosphere. All chemicals used were reagent grade. Heavy metal analysis was by inductively coupled plasma (ICP) spectroscopy, employing four-point standard calibration prior to and following analysis of unknowns, and regular triplicate analyses to confu'm the method. Analytical controls were maintained throughout, including use of control and blank samples, mass balance checks, and minimization/accounting for losses incurred in transfer or solid separation steps. Equilibrium S t u d i e s Sorption isotherm studies were conducted in well-sealed, ] 25-ml polypro-

pylene bottles, which, when agitated, can be assumed to function as completely mixed batch reactors. Adsorption equilibria were assessed by two experimental approaches. In the first, varying amounts of prepared iron sorbent were contacted with 100 ml of aqueous solution containing the metal

Table I. Physical Properties of Iron Sorbent

Sample 200/325 mesh 60180mesh 30•40 mesh

Density (g/cnP)

BET SurfaceArea (m21g)

Pore Volume(cnP/g)

3.640 2.536 6.272

1.336 1.109 0.036

0.00533 0.00648 0.00564

© Copyright Elsevier Science Inc.

13

Table II. Freundlich and Langmuir Model Coefficients for Cadmium, Zinc, and Lead Adsorption onto Shot-Blast Fines

FreundlichModeP Mesh Size

LangmuirModeP

CO(mg/L)

pH

Ic

KF

n

QO

5.0 5.0 5.0 5.0 5.0 25.0 25.0 25.0 25.0 5.0 25.0 25.0

4.0 5.5 5.5 5.5 7.0 5.5 5.5 7.0 7.0 7.0 5.5 7.0

0.01 0.001 0.01 0.1 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

4.66 7.60 6.91 5.70 7.96 7.08 7.75 9.05 7.50 19.72 23.29 27.41

0.202 0.081 0.071 0.128 0.105 0.066 0.188 0.213 0.151 0.143 0.155 0.128

5.07 8.10 7.36 6.18 8.67 8.19 12.96 14.13 10.29 21.07 33.99 37.25

47.91 72.50 86.85 62.40 66.47 71.75 1.27 3.62 11.20 93.43 3.26 7.19

5.0 5.0 5.0

4.0 5.5 7.0

0.01 0.01 0.01

10.88 15.38 13.55

0.132 0.196 0.197

12.34 18.98 17.10

33.50

5.0 5.0

4.0 5.5

0.01 0.01

18.54 28.89

0.708 0.655

59.88 65.019

Cadmium

200/325 200/325 200/325 2001325 2001325 200/325 60/80 60/80 60180o 30/40 30/40 30/30 Zinc

2001325 200/325 200•325

6.86 6.84

Lead

2001325 200/325

0.486 0.897

• Based on qo = K~Cff where C, is in moJL and q, is in mg/g. ~Based on q° = O:oCJ(1 + bCJ where C, is in moJL and q, is in mg/g. qonic strength of background using NaCIO4. aSackground solution also contained 40 ppm Ca~'+.

of interest at pre-established conditions of pH, ionic strength, and total dissolved contaminant concentration, C o. Following a 48-72 hour reaction period on a rotary tumbler, the equilibrium pH was measured and a sample taken from each reactor and filtered to separate the solid. The aqueous samples were acidified with 1:1 HNO3 and analyzed for equilibrium liquid phase concentration, C e, using an ICP spectrometer. Two or three reactors containing no sorbent were included to establish the initial concentration of sorbate. Solid-phase loading of metal, Qo was then computed from the mass balance, Q~ = (CO - C j D , where D represents the dose of sorbent in the reactor in g/L. By this procedure, the relationship between liquid- and solidphase equilibrium concentrations was developed for the given set of solution conditions, most notably a single initial pH value for the wastewater. Although the pH will tend to drift from its designated value as the solution reacts with the solid phase, this approach yields useful information because it mimics the typical pattern of treatment systems. A second set of isotherm experiments followed an identical pattern except that: (1) all samples had the same mass of sorbent, and (2) the initially adjusted pH of individual samples was varied between about 3.5 and 10.5. METAL FINISHING • NOVEMBER 1995

This approach enables an in-depth evaluation of pH effects on metal remoral, especially important for metal adsorption systems. A repeat experiment without sorbent provided observation of metal-solubility/precipitation phenomena for a particular solutionphase condition. For both isotherm approaches, the solution pH was adjusted to the desired level(s) and the samples sealed and set overnight. The following day the pH was re-adjusted, if necessary, noting any pH drift, before proceeding with the reaction period.

Batch Rate Studies

Adsorption kinetics were investigated in a continuously stirred, 2-L

stainless steel beaker with cover. Just prior to adding the iron fines to the stirred reactor, two unfiltered and two filtered aqueous samples are taken from the reactor to verify the initial target metal concentration and identify any filter losses of solute. A predetermined amount of iron was added to the test solution at time zero, and 5-ml samples were extracted from solution and filtered through a prewashed glass fiber filter to separate the solid. The solution was then analyzed for the target metal by ICP spectroscopy. By a mass balance assumption, the difference between the metal remaining and that initially present is that adsorbed

onto the sorbent. The pH of the solution was also monitored throughout, as it is suspected that water-sorbent interactions, uptake of metal from solution, and exposure to the atmosphere would result in a measurable pH drift. RESULTS Adsorption Modeling

Equilibria and

Experimental equilibria can be described well by common expressions such as the Freundlich and Langmuir models. Model constants are presented for varying solution conditions in Table II. Trends in sorption capacity are generally captured in the Freundlich Kf and Langmuir QO respectively. For instance, isotherm studies with different total Cd(II) concentration indicate an increase in adsorption capacity with increased total sorbate concentration, Co. Comparing the 5 ppm versus 25ppm studies at pH 5.5 for 200/325 mesh fines and at pH 7 for 30/40 mesh sorbent, K/and QO values are higher for the 25 ppm case. Surface heterogeneity of the solid, and increased driving force between the bulk solution and the surface for higher C O, account for this trend, which is opposite that reported in some studies of metal removal onto iron oxides where the adsorption isotherm would be classified 15

9661. EF:ISIN=IAON - DNIHSINI-I 7V..L=71N

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Figure 2. Adsorption edges for (a) Cd(ll); (b) Zn(ll); and, (c) Pb(ll) "No sorbenff curve is experimental removal by precipitation without sorbent present METAL FINISHING

• NOVEMBER

1995

beds may be an effective mode of application for metal removal. Several comparative isotherms were conducted for the three test metals with three commercial activated carbons. The results demonstrate that the shot blast compares favorably, with adsorption capacities exceeding those for the carbons, especially for the granular fractions. Adsorption capacities in the designated concentration range are also greater than literature values for various brands of activated carbons, and are comparable t o those reported for various metal oxides, including zeolites, and some hybrid oxides and chelating polymer exchangers. 7"!o-12 As suggested previously, surface complexation is a proposed mechanism for heavy metal removal onto the recovered iron sorbent. By virtue of its composition, the material is assumed to have oxide or hydroxylated surface sites in solution such as >XOH2 +, >XOH, and XO- representing positively charged, neutral, and negatively charged sites, respectively. The symbol, >X, represents the surface oxide base element; in this case, typically Fe and possibly Si due to the presence of "impurities" such as sand particles from castings. An example of surface complexation of divalent metal cations, Me2+, is:

(I)

Equation 1 represents unidentate surface complexation and incorporates the impact of pH on adsorption equilibria. Metal-ligand complexes such as MeOH + can also be adsorbed in a manner similar to that expressed by Equation l, the distribution of which are also controlled by the pH of the system. It is also likely that oxidation-reduction behavior is conwibuting to metal uptake. A majority of the recycled fines are ground derivatives of steel shot and thus have a metallic core, primarily Fe(s). A hydrous oxide layer forms over time, probably in a heterogeneous fashion, the nature of which is impacted to some degree by solution conditions. It is postulated, therefore, that some metal pollutant molecules are reduced by direct contact with Fe(s), particularly during the earlier stages of a run before the formation of an extensive oxide layer. An example 19

50

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Ce (mg-Cd/L)

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(2)

More detailed studies of the material properties and behavior under various aqueous-phase conditions are required to better understand the metal removal mechanisms involved. Batch Kinetics and M o d e l i n g Laboratory-scale batch rate analyses can be used to evaluate kinetic and other parameters for the design of batch mode treatment systems given the desired treatment goals. Figure 4 depicts the time rate of removal of Cd and Pb at pH 7 and 4, respectively, for 0.2 g/L dosage of 200/325 mesh shotblast fines. It is evident that the large percentage of metal removal is achieved in 5-10 hours. As a result, batch-mode isotherm studies were reacted for 48-72 hours to assure attainment of virtual equilibrium for 200/ 325 mesh material. Based on the given sorbent dosage of the sample, the overall metal removal after this period corresponds well with the respective isotherm values represented previously. An attempt has been made to simulate batch rate data using a dual-rate mass transfer model that includes external film diffusion and lumps any intraparticle diffusion processes into a single surface diffusion term. Models of this

In Equation 6,

type have been used for simulating the dynamic removal of pollutants on other porous and slightly porous adsorbents, including granular iron oxide hybrids. ]3'14 The material balances comprising the dual-resistance model used in the simulations are given here in dimensionless form. The liquid phase material balance for a single solute is (symbols are defined in the N o -

1~

,

I

DiDs'r b E d =

(10)

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I

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l

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20

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40

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Time (Hours) Figure 4. Batch rate data and model simulations for Cd(ll) and Pb(ll) adsorption on 2001325 mesh fines.

v

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METAL FINISHING • NOVEMBER 1995

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PHo= 4.0

4 0

I

I

I

I

I

I

I

10

20

30

40

50

60

70

Time (Hours) Figure 5. Effect of pH on batch rate adsorption data of Cd(ll) and pH drift over course of experiment for varying initial pH. 0.5 g/L of 200/325 mesh iron sorbent; Co = 5 ppm Cd(ll); 0.01 M ionic strength.

The model is used to simulate several variable conditions of metal uptake; namely metal type, initial solution pH, sorbent dose, and particle size of sorbent. In each case, the model is able to simulate batch data reasonably well, except for the portion of several curves between about 10 and 30 hours when, after initially high and rapid removal of cadmium, there is a momentary increase in concentration of the sorbate before resumption of slow continuous uptake of metal. The top portion of Figure 5 illustrates the impact of the influent pH of the wastewater on the rate of removal 22

of cadmium in a mixed batch reactor. As in the isotherm studies, the extent of removal tends to increase, with pH with only a small difference in removal between pH 5.5 and 7. This can be explained, at least in part, by the pH drift of respective test samples illustrated in the bottom portion of Figure 5. Although they begin 1.5 pH units apart, the pH of the pHi,,iaa t 7 and 5.5 solutions nearly converge after 24 hours to a value between 6.5 and 6.7. The pH 4 solution increases by more than 2 pH units initially before leveling off at pH - 5 . 5 . These trends are similar to those observed in the equilib-

rium experiments, illustrating further the acid-base characteristics of the sorbent in aqueous solution. Figure 6 illustrates that this pattern is essentially repeated for Zn(II) uptake for an initial pH of 5.5. External, k9 and intraparticle, Di, diffusion coefficients were determined by a best-fit calibration of the dynamic model with batch rate data and are given for several cases in Table III. These results suggest that the rapid rate of initial removal of metal, characterized by the external rate coefficient, kf, is not a function of pH, dose, or particle size. Second, although sorbent dose obviously impacts the extent of removal of cadmium, it has no impact on the rate of removal for a given solution-phase condition. For the powdered (200/325 mesh) fraction, intraparticle mass transfer appears to decrease with decreasing pH. Due to the fairly rapid attainment of equilibrium for the powdered fraction, however, the model curve is only moderately sensitive to changes in the D i value given for these cases in Table III. For larger particle sizes, the initial removal is also rapid as seen in Figure 7 and from the kf values in Table III. D i values, however, are approximately an order of magnitude less than for powdered fraction cases (e.g., compare the pH 7 cases for Cd), with model sensitivity to D; much greater for the larger fines. Finally, although external mass transfer rates are essentially equal for the metals studied, the magnitude of intraparticle mass transfer rate, D,, is much greater for lead than for either cadmium or zinc.

CONCLUSION

The principal findings of this work are the following. i. Recovered shot-blast fines have a favorable adsorption capacity for heavy metals, with removals greater than activated carbons and comparable to those of natural and hybrid metal oxides and some chelating polymer exchangers. 2. For the three metals tested, adsorption capacity is the greatest for lead, followed by zinc and cadmium. Adsorption equilibria can be adequately described by simple isotherm models such as the METAL FINISHING • NOVEMBER 1995

1

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10

20

30

40

50

60

70

O

0

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Time (Hours) Figure 6. Batch rate data and model simulations for Zn(ll) adsorption and pH drift over course of experiment for initial pH = 5.5. 200/325 mesh iron sorbent; 0.01 M ionic strength.

.-. O o

1

I

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100 125 150 175 20O

Time (Hours) Figure 7. Batch rate data and model simulations for cadmium adsorption by various particle sizes.

6.

Freundlich and Langmuir equations. As anticipated, adsorption of metals is very sensitive to pH, with adsorption generally increasing with pH. The pattern of increase varies for individual metals as driven by solution phase equilibria of oxide/hydroxide complexes and precipitates. Adsorption is moderately sensitive to the ionic strength of the solution with adsorption capacity decreasing for increasing background ionic concentration and the presence of inorganic cations such as Ca 2+. Together with other solution.phase characteristics of the sorbent, these results suggest that surface complexation may be an important mechanism of metal uptake by shot-blast fines. Redox reactions likely play an important role in metal removal as well. Adsorption rates are rapid for batch systems using powdered (i.e., 200/ 325 mesh) fines, with 90% or more of the removal occurring within 5 to 10 hours depending on the system. In tests using larger particle sizes, there was rapid initial removal followed by a slow and lengthy approach toward the equilibrium position. Preliminary simulations using an existing dual resistance dynamic model demonstrate that it may be suitable for describing reactor performance, thus qualifying as a useful design tool for a field-scale process. Rate parameters evaluated by calibration of the model suggested little impact of sorbent dose and, arguably, moderate impact of pH on adsorption rate. As anticipated, sorbent particle size influences metal uptake kinetics, with increasing size decreasing the intraparticle mass transfer rate. Intraparticle rates for lead were an order of magnitude

Table III. Model Rate Coefficients for Bath Rate Simulations

Mesh Size of Sorbent 2001325 2001325 2001325 2001325 60/80 30140 200/325 2001325

24 •

Metal

Co (rag~L)

pH

Dose (g/L)

kt (cm/sec)

D~(cm2/sec)

Cd Cd C,d Cd Cd Cd Zn Pb

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

4.0 5.5 7.0 7.0 7.0 7.0 5.5 4.0

0.5 0.5 0.5 0.2 0.5 0.2 0.3 0.2

4.5 x 10- s 5.0x 10-3 4.5x10 -3 5.0x10 -3 4.0x10 -3 3.9x 10-3 4.0 x 10- ~ 4.8x 10-3

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METAL FINISHING

° NOVEMBER

1995

greater than those for cadmium and zinc. Considering the expense of commercial and synthetic adsorbents, and that the use of the iron fines offers the possibility of recycling an otherwise waste product, the development of a process for treating aqueous-phase heavy metal wastes offers potential environmental and economic benefit for industrial pretreatment and hazardous waste remediation. Results to date suggest that the process should be most effective for metal removal in the low to medium mg/L range and, for cationic metals, at pH 5 or higher. Successful application requires nonhazardous ultimate disposal of the fines. Substantial metal recovery has been achieved in preliminary tests using chemical regeneration methods at reduced pH. Continuing investigations include evaluating other metal recovery schemes, using granular sized fines in fixed-bed mode, sorbent preprocessing methods for enhancing process performance, the effects of lot variability and storage, and the impact of the presence of competing and/or complexing substances.

Acknowledgment The laboratory assistance of Claude Ramos and Sabyasachi Chatterjee is appreciated. This material is based in part upon work supported by Tyler Pipe, Inc. of Tyler, Texas, the Texas Advanced Technology Program under grant no. 003613009, and the U.S. EPA Risk Reduction Engineering Laboratory under Cooperative Agreement no. CR 821824-01-0. The views presented do not necessarily represent those of the supporting agencies and the mention of trade names does not constitute endorsement. A patent on the processes described herein is pending.

Nomenclature b = Langmuir isotherm constant (L3/ M-pollutant) C = liquid-phase concentration (Mpollutant/L 3) Cs = liquid-phase concentration at particle surface (M-pollutant/L 3) Dg -- solute distribution parameter E a = surface diffusion modulus Kf = Freundlich isotherm constant [(L3)" (M-pollutant)l - n/M-solidi METAL FINISHING • NOVEMBER 1995

kf = external film diffusion coefficient (L/t) n = Freundlich isotherm exponent q = solid-phase concentration (M-pollutant/M-solid) Q" = Langmuir isotherm constant (Mpollutant/M-solid) R = carbon particle radius (L) r = radial distance in carbon particle (L) Sh b = Sherwood number for batch system t = t i m e (t)

10.

11.

12.

v = reactor volume (L 3) W C = mass of sorbent (M-solid) = particle void p = density of carbon particle (M-solid/ L 3) % = duration of rate experiment (t)

13.

References 1. Knocke, W.R. et al., "Removal of Soluble Manganese by Oxide-Coated Filter Media," Journal of the American

Water Works Association, 83(8):64--69; 1991 2. Edwards, M. and M.M. Benjamin, "'Regeneration and Reuse of Iron Hydroxide Adsorbents in Treatment of Metal-Bearing Wastes," Journal of Water Pollution Control Federation, 61:1524--1533; 1989 3. Brewster, M. et al., "Electrochemical Iron Generation: The Ideal Process for Simultaneous Removal of Heavy Metals from Contaminated Groundwater." In Tedder, D.W., ed., Proceedings, Emerging Technologies in Hazardous Waste Management V, American Chemical Society, Washington; 1993, pp. 315-318 4. Budilovski, J., "New Industrial Wastewater Treatment System," The Economist, 2:49-52; 1990 5. Zachara, J.M. et al., "Chromate Ad-sorption on Amorphous Iron Oxyhydroxide in the Presence of Major Groundwater Ions," Environmental Science and Technology, 21:589-594; 1987 6. Smith, E.H. et al. "Sorption of Heavy Metals by Lithuanian Glauconite," Water Research, in press; 1995 7. Gao, Y. et al., "A New Hybrid Inorganic Sorbent for Heavy Metals Removal," Water Research, 29:2195-2205; 1995 8. Cowan, C.E. et al., "Cadmium Adsorption on Iron Oxides in the Presence of Alkaline-Earth Elements," Environmental Science and Technology, 25: 437-446; 1991 9. Davis, J.A. and D.B. Kent, "Surface Complexation Modeling in Aqueous Geochemistry." In Hochella, M.E Jr., and A.E White, eds., Reviews in Mineralogy-Volume 23: Mineral-Water In-

14.

terface Chemistry, Mineralogical Society of American, Washington.; 1990, pp. 177-260 Corapcioglu, M.O. and C.P. Huang, "The Adsorption of Heavy Metals onto Hydrous Activated Carbon," Water Research, 21 : 1031-1044; 1987 Kesraoui-Ouki, S. et al., "Effects of Conditioning and Treatment of Chabazite and Clinoptilolite Prior to Lead and Cadmium Removal," Environmental Science and Technology, 27:11081016; 1993 Sengupta, S. and A.K. Sengupta, "Characterizing a New Class of Sorptive/Desorptive Ion Exchange Membranes for Decontamination of HeavyMetal-Laden Sludges," Environmental Science and Technology, 27:21332140; 1993 Smith, E.H., "Modified Solution of Homogeneous Surface Diffusion Model for Adsorption," Journal of Environmental Engineering, 117:320-338; 1991 Theis, T.L. et al., "Evaluating a New Granular Iron Oxide for Removing Lead from Drinking Water," Journal of the American Water Works Association, 84(7):101-105; 1992 MF

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