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Microtox™ characterization of foundry sand residuals
WASTE MANAGEMENT Waste Management 18 (1998) 227±234
Microtox2 characterization of foundry sand residuals K.C. Bastian a, J.E. Alleman b* a US Geological Survey,Lawrence, KS 66049-3839 ,USA School of Civil Engineering Purdue University,West Lafayette, IN 47907-1284 USA
b
Received 9 July 1998; received in revised form 8 May 1998; accepted 20 May 1998
Abstract Although foundry residuals, consisting mostly of waste sands, represent a potentially attractive, high-volume resource for bene®cial reuse applications (e.g. highway embankment construction), prospective end users are understandably concerned about unforeseen liabilities stemming from the use of these residuals. This paper, therefore, focuses on the innovative use of a microbial bioassay as a means of developing a characterization of environmental suitability extending beyond the analytical coverage already provided by mandated chemical-speci®c tests (i.e., TCLP, etc.). Microtox2 bioassays were conducted on leachates derived from residuals obtained at a wide range of facilities, including: 11 gray and ductile iron foundries plus one each steel and aluminum foundries. In addition, virgin sand samples were used to establish a relative `natural' benchmark against which the waste foundry sands could then be compared in terms of their apparent quality. These bioassay tests were able to eectively `®ngerprint' those residuals whose bioassay behavior was comparable to that of virgin materials. In fact, the majority of gray and ductile iron foundry residuals tested during this reported study elicited Microtox2 response levels which fell within or below the virgin sand response range, consequently providing another quanti®able layer of support for this industry's claim that their sands are `cleaner than dirt.' However, negative Microtox2 responses beyond that of the virgin sands were observed with a number of foundry samples (i.e. four of the 11 gray or ductile iron sands plus both non-iron sands). Therefore, the latter results would suggest that these latter residuals be excluded from bene®cial reuse for the immediate future, at least until the cause and nature of this negative response has been further identi®ed. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Waste foundry sand; Bioassay; Microtox2; Waste reuse; Environmental quality
1. Introduction Each year, the US foundry industry utilizes over 5 million tons of sand in expendable mold casting operations [1]. In these operations, molten metal is poured into molds made of sand which has been shaped and hardened in conjunction with various additives to withstand the pressure and heat derived from the heated metal. After the metal has cooled, the sand is separated from the casting and foundries then go to great lengths to recycle as much sand as possible. Some wastage after each casting cycle is necessary, though, due to physical and chemical deterioration of the sand and to the fact that virgin sands are required in preparing certain sections of the mold. The vast majority of the wastes is sand, usually silica-based, although some olivine, zircon, chromite, and other base sands are used [2]. However, these wastes also contain residues of various * Corresponding author. Tel.: +1-765-494-7705; fax: +1-765-4961107; e-mail: [email protected]
casting components, including clays, coal and petroleum derivatives, and other organic chemicals. A schematic of foundry processes and material ¯ows is presented in Fig. 1. The constructive reuse of these high-volume foundry residuals represents a decidedly bene®cial goal with distinct economic and environmental bene®ts. Many potential end users, however, are reluctant to use these residuals given an inherent concern about unforeseen liabilities [3]. These wastes have already been extensively tested in compliance with current regulations regarding leachate characterization (i.e. using leachate generation and testing protocols such as the TCLP), and the available results strongly suggest that many gray and ductile iron foundries are discarding residuals whose quality is fully amenable to their future use with a variety of bene®cial reuse applications such as highway embankment construction [4±7]. In fact, given the preponderance of data showing low contaminant leachate levels, these wastes have been optimistically quali®ed by industry spokesmen as having less potential impact on ground-
0956-053X/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved PII: S0956 -0 53X(98)00030-0
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K.C. Bastian, J.E. Alleman/Waste Management 18 (1998) 227±234
Fig. 1. Schematic of foundry processes and material ¯ows.
water than do virgin soils [8]. In spite of this industry's data and their related claims about `cleanliness,' though, lingering uncertainties still persist regarding the longterm behavior of these materials in terms of their potential release of chemical contaminants not covered by current testing. As such, these materials are commonly land®lled at considerable expense to the responsible foundries, while at the same time unduly consuming precious land®ll volume. This project was consequently developed to provide a complementary assessment protocol based on surrogate biological indicators which could be used to eectively `®ngerprint' those residuals whose bioassay behavior was comparable to that of virgin materials. Quite frankly, foundry waste sands can be exceedingly complex in terms of the potential composition and character of their leachates. The dominant concern largely revolves around organic residues found within the waste sand, stemming from the original use of complex sand binders or, perhaps, from byproducts created from these latter compounds during the casting process. These `synthetic' additives, used in conjunction with a diverse group of `natural' materials (including clays, sea coal, and wood derivatives) provide the requisite surface and strength properties for the involved mold and core casting components. In many instances, the casting temperatures associated with ferrous metals are suciently high to thoroughly `burn out' these mold and core organics. However, with various types of casting
techniques and/or binding additives, it would appear that some of the waste sands still contain a diverse range of undesirable organic contaminants. Mandating chemical-speci®c testing for each of these potential contaminant species would be exorbitantly expensive and, even then, these individual tests would provide no information about potential complications caused by problematic synergisms. Leachate bioassay testing, though, would provide an all-encompassing characterization of this material's potential to impose a negative environmental impact, thereby generating an exceedingly useful assessment of a waste's fundamental quality and suitability as compared to, and judged against, virgin sands. Extending beyond chemical-speci®c leachate testing (e.g. TCLP), our research group consequently developed a foundry residual leachate generation and testing protocol based on the Microtox2 bioassay system. This test uses light producing bacteria to characterize the presence of inhibiting contaminants, as measured by the resultant suppression of their light-emitting activity. In turn, these results provided a biological assessment of foundry wastes in terms of their environmental character. Although the commercial name `Microtox2' inherently implies that `toxicity' is being evaluated, the relative meaning of any such `toxic' indication is subject to considerable interpretation. All of the tested residuals during this project had, in fact, already been proven to be `non-toxic' (by a considerable margin) according to
K.C. Bastian, J.E. Alleman/Waste Management 18 (1998) 227±234
the applicable regulatory criteria. The involved concept of using a Microtox2 test to evaluate these foundry residual leachates, therefore, is moreso focused on qualifying a bioassay response relative to virgin materials. For those samples which fell within the response range established for clean, natural sands, their Microtox2 behavior provided a strong indication that these residuals are suitable for subsequent reuse. Conversely, for those residuals whose Microtox2 response showed excessive negative impacts (i.e. decreased light emission as compared to virgin sands), these results suggest that their exclusion from further consideration for bene®cial reuse would be warranted until the causative factors can be established. This relatively simple and inexpensive test was correspondingly designed to evaluate each waste residual in comparison to the relative bioassay behavior of virgin materials. As documented within this report, the `cleanliness' of the residuals obtained from many foundries was readily evident. Leachates from seven of the iron foundries tested caused less inhibition of light production by the Microtox2 bacteria than did virgin sands. Taken literally, it appears that these residuals are, truly, `cleaner than dirt.' Furthermore, for these wastes, no real dierences were seen between system sands and fresh or aged waste sands. However, in a limited number of instances, there were clear and consistent indications that the tested foundry wastes had released an inhibitory contaminant into the leachate waters, thereby resulting in a quanti®able depression in observed microbial activity. For four iron foundries, the Microtox2 responses to fresh waste sand leachates were noticeably less positive than to system sands, and these responses generally varied more than those elicited by residuals from the seven `clean' foundries. Residuals from the steel foundry and the aluminum foundry also displayed inhibition of light production which was consistently greater than that caused by virgin sand leachates. Overall, therefore, this innovative bioassay test appears to oer an ecient and expedient approach to `®ngerprinting' foundry locations for which constructive waste sand reuse could subsequently be pursued without undue concern about negative environmental impacts. 2. Materials and methods 2.1. Sample procurement One-half- to 2 kg samples of system and fresh waste sand were collected by foundry or university personnel from a total of 13 foundries for testing. These included 10 iron greensand (e.g., using primarily clay-based binders) foundries, one iron foundry which used organic chemicals as the primary binders, one steel greensand foundry, and one aluminum greensand foundry. In
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addition, aged waste sand samples were obtained from two of the iron greensand foundries, the steel foundry, and the aluminum foundry. Between six and 37 samples were obtained from each facility over periods ranging from 5 days to 6 months. Finally, 13 virgin sand samples, donated by foundries or sand manufacturing companies, were also subjected to bioassay testing. The majority of these samples were packed in glass jars, cans, or zipper storage bags and returned to Purdue University for storage at 4 C prior to eventual testing. All samples were labeled with the appropriate information to identify sample location, type, and date. 2.2. Leachate generation From each stored sample, 20.000.05 was taken and placed in a ¯ask containing 801 ml of a 2% NaCl solution. As compared to the relative level of solids used in the TCLP test, therefore, the solids to liquid ratio used during these leachate studies (i.e. 20:80) was fourfold higher (i.e., thereby being more conservative ). During the development of various testing regimes with the Microtox2 90% Comparison Test (see following discussion), the use of a saline leachant was considered necessary in those cases where the bacteria were actually added directly to the leachate suspensions (to simulate the natural, marine environment of the Microtox2 organisms). Although the ®nal leaching strategy did not include these bacteria, this saline leachant protocol was continued as yet another conservative measure, by way of prospectively enhancing the release of bound contaminants (e.g. metals released by way of ion exchange, etc.). Each ¯ask was covered with para®lm, manually agitated to break up clumps, and placed on a shake table for 182 h. The solution was then allowed to settle and 30 to 40 ml of supernatant was poured into polycarbonate centrifuge tubes. The samples were centrifuged for 16 min at 10,000 rpm (approximately 10,000 g). The centrifuge supernatants were ®ltered through 1.5 mm pore size glass ®ber ®lters. The pH was then recorded and, when necessary, a phosphate buer solution was used to adjust pH to a level between 6.5 and 8.0. Samples were analyzed immediately or transferred into borosilicate glass vials, covered with parafilm, capped, and stored at 4 C for no more than 72 h. The control tests were conducted with the same NaCl solution, but without any sand. 2.3. Microtox2 testing protocol The Microtox2 90% Comparison Test used during these tests to determine foundry sand toxicity was recommended and developed by the involved commercial vendor speci®cally for samples believed to contain low levels of contaminant toxicity [9]. As mentioned
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previously, this test uses a special light emitting bacterial (Vibrio ®scheri) suspension whose energy emission in the presence of unknown samples can then be quanti®ed within a darkened chamber using a photodetector. The reported results derived with this test subsequently represent the percentile decrease in light emission measured between the control (®ve each) and sample (®ve each) cuvettes. Short- and long-term testing protocols were initially devised for this test, and in some instances the bacteria were actually added to the shaken leachate suspensions. However, the results presented in this paper were all obtained using a ®nal testing strategy in which the bacteria were added after the multi-hour leaching step. In each test, 5 leachate replicates (i.e. derived from 182 h of contact with the tested samples) were compared with ®ve control replicates consisting of unamended 2% NaCl solution. The actual process of conducting any given test can be summarized as follows. The freezedried Microtox2 bacterial culture was initially reconstituted and, in each of ten cuvettes, the light produced by 0.1 ml of this bacterial suspension was measured. A 0.9 ml sample was then added to ®ve cuvettes and 0.9 ml control diluent was added to the other ®ve cuvettes. Light output from each cuvette after 5 and 15 min was measured, and the percent dierence between initial light output and ®nal light output was quanti®ed. The replicate values were averaged and an overall toxicity value and range of certainty were calculated. For quality control purposes, complementary bioassay tests were conducted on an approximately monthly basis using reference toxicants. A known organic toxicant (10 mg/l phenol) was used for this purpose, thereby verifying the repetitive sensitivity of the employed Microtox2 cultures. 2.4. Additional leachate testing In addition to routine measurements of the their pH levels, representative leachates from each of the tested foundries were also evaluated in terms of their resultant chemical oxygen demand (COD) concentrations (using HACH dichromate digestion tubes). The latter COD tests were conducted on ®ltered leachates, such that they represent soluble concentrations (i.e. sCOD).
important to remember that the Microtox2 response data is not being used to de®nitively calibrate toxicity, but to identify and `®ngerprint' those residuals whose quanti®able character appears commensurate with that of natural materials (i.e. perhaps not ``cleaner than dirt,'' but eectively indistinguishable)! 3.2. pH Virgin sand pH values averaged approximately 6.9, being slightly acidic. Iron greensand foundry residual leachates were usually slightly basic, although pH levels ranged from 6.5 to 10.1. The chemically bound waste foundry residual leachates had neutral pH levels, ranging from 6.8 to 7.3. Leachates from aluminum foundry waste residual leachates were slightly basic. Steel foundry waste residual leachate pH levels were not measured. No apparent correlation existed between initial pH and bioassay response. 3.3. Microtox2 Fig. 2 exhibits Microtox2 5-min response data for the virgin sand samples. In this ®gure, each data point represents the response of one sample (i.e. percentile values indicating the loss of light emission between the control and sample tests), with vertical error bars representing the estimated error of each measurement. 15-minute response data closely paralleled the data shown. For the 13 samples, mean inhibition of light production by virgin sand leachates was 9.0 and 9.2% for 5- and 15-minute readings, respectively, with standard deviations of 7.7 and 6.9%. A solid horizontal line is used to represent the mean responses, while dotted horizontal lines indicate one standard deviation above and below mean values. As response to virgin sands is the standard to which responses to foundry residuals will be compared, similar horizontal lines indicating mean and standard deviation of responses to virgin sands were also included in all ®gures displaying bioassay data.
3. Results and discussion 3.1. Overview Data were obtained for 13 virgin sand samples from various sources and for samples from eleven gray and ductile iron foundries, one steel foundry, and one aluminum foundry. Measurements included leachate pH values and 5- and 15-min Microtox2 response data. It is
Fig. 2. Virgin sand MicrotoxTM (5-min) response.
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Again, these latter percentile response levels are relative to a `clean' control water whose known composition would not have been expected to exert a negative impact on the employed bacteria. By comparison, the virgin sand leachates engendered a range of bioassay eects which on average were marginally `inhibitory' (i.e. with higher percentile dierences in light production for the control versus sample tests) due to an unknown stress. While the magnitude of this impact was not at all extreme, there was no readily obvious reason why the virgin sands should behave in this fashion. The fact remains, though, that these virgin sand results qualify as the best natural benchmark against which the foundry residuals can directly be judged. Fig. 3 displays a summary of the 5-min Microtox2 response levels observed with residuals from all 13 foundry facilities. Here again, these bioassay results (depicted on the y-axis) are quanti®ed as a percentile decrease in light output for test microorganisms exposed to sand leachates as compared to those exposed solely to the specially prepared `control' water. Each data point depicts the mean response for all samples from the waste type it respectively represents. The range of response values from each sample type is shown by the vertical bars passing through the data points. Types of data include system sands (e.g. sands which have been prepared to be used as mold sands), fresh waste sands, aged waste sands, and air handler dusts (e.g. ®nes resulting from the physical breakdown of mold sands). It is seen that the Microtox2 response varied greatly. The bioassay response by residuals from 11 of eleven iron foundries (F1 through F7) was consistently below the average response to virgin sands. Residuals from three of the iron foundries (F8 through F10) produced
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inconsistent responses. Sands generated by the single foundry (F11) which utilized a synthetic organic mold sand binder, rather than a clay-based `greensand' process, displayed a consistently inhibitory eect on the Microtox2 organisms. The responses to fresh waste sands from the steel foundry (N1) and all residuals from the aluminum foundry (N2) demonstrated signi®cant bacterial inhibition. Only one aged waste sand sample from the steel foundry was available; Microtox2 response to leachate from this sample was below the virgin sand average response. An example of data from ``clean'' residuals is shown in Fig. 4. Microtox2 responses (5- and 15-min) to individual fresh waste sand samples taken from this foundry (Foundry F3) over a period of 6 months are shown. In this ®gure (as in the remaining Microtox2 response ®gures), each data point represents the response of one sample, with error bars representing the estimated error of each measurement. Two points are worthy of note. First, no clear pattern of increase or decrease in inhibition of light production is seen over the sampling period. Second, the 15-minute Microtox2 responses are not signi®cantly greater than the 5-min responses. As V. ®scheri response to metals is not generally observed within 5 min, this latter observation suggests that organics, rather than metals, are causing the bacterial inhibition. As suspected, therefore, chemical-speci®c testing which focuses on metals or other inorganics (as does the presently mandated characterization) may not provide an eective estimate of potential environmental impact. Indeed, Microtox2 testing provided no indications of metals-related inhibition for any of the residuals tested. While the response to Foundry F3 fresh waste sands was consistently within the range established by virgin
Fig. 3. MicrotoxTM response to foundry residuals after 5 min.
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sand testing, samples from other greensand foundries evoked wide variations in Microtox2 response (see, for example, Fig. 5). For this foundry (F10) (and that of the inhibition seen with Foundry F9 as well), it is probable that the bacterial inhibition was caused by the particular core binder chemicals, including so-called `hot box' binders, which were used. The reason(s) for the dierences in inhibition over a period as little as 1 week, however, cannot be determined without additional testing; possibilities include variations in sampling procedures or in casting processes. A consistently high level of bacterial inhibition was observed with samples from the foundry which used an organic chemical (rather than greensand) binder system for its mold sands (Foundry F11; see Fig. 6). The samples from this foundry had a dierent color than most of the other residual samples and had a distinct organic chemical odor, likely due to binder chemicals remaining on the residuals after casting. One major dierence between iron and steel casting is casting temperature (1565 C for steel, as opposed to approximately 1370 C for iron). It might be hypothe-
sized that any contamination due to organics would be decreased, due to the higher temperature. Surprisingly, however, the inhibition of light production by Microtox2 organisms exposed to steel foundry fresh waste sand leachates was signi®cantly greater than the bacterial response to virgin sand leachates (see Fig. 3). This behavior could stem from the use of particular binders or from the use of greater quantities of applied binder chemicals than are used for iron casting. Alternatively, due to the large casting sizes produced by this foundry (up to 4000 kg), the cores may have been of such large size that inner core temperatures were not high enough to oxidize all binder chemicals. Aluminum is cast at a much lower temperature than iron (approximately 700 C). Consequently, the molds and cores break down to a much smaller extent during casting, necessitating the use of additional vibration or shaking to separate the sand from the castings after the metal has cooled. Much higher levels of organic contamination of aluminum foundry residuals were expected, and the results of Microtox2 testing were consistent with such expectations: bacterial inhibition levels by
Fig. 5. Foundry F10 fresh waste sand MicrotoxTM (5-min) response.
Fig. 4. Foundry F3 fresh waste sand MicrotoxTM response.
Fig. 6. Foundry F11 fresh waste sand MicrotoxTM (5-min) response.
K.C. Bastian, J.E. Alleman/Waste Management 18 (1998) 227±234
system, fresh waste, and even aged waste sand leachates were consistently higher than had been seen in iron or steel foundry sands (see Fig. 3). 3.4. COD There was a sizable variation in the levels of sCODs measured within the waste foundry sand leachates derived during this study. At the low end of this spectrum, the virgin sands had the lowest soluble COD values, with a range of about 2±24 mg/l. Furthermore, the foundries which exhibited the best (i.e. least inhibitory) levels of Microtox2 response also had the lowest observed sCOD values. Foundries F1 and F2 samples had respective sCOD ranges of 25±28 and 50±65 mg/l. At the other extreme, there did appear to be a correlation between higher levels of Microtox2 response and leachate sCOD. For example, Foundry F9 had a sCOD range of 125 to 160 mg/l, and inhibition levels consistently above the virgin sand benchmark. Similarly, Foundry F11 had an sCOD range of 70±130 mg/l, and an inhibitory percentile of about 60%. Lastly, Foundry 10 had the widest range of observed inhibition with its fresh and aged sands, and here again the corresponding sCODs also had the highest level of observed variation (ranging from 25 to 210 mg/l).
.
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.
.
4. Summary The ®ndings and corresponding observations derived from this study can be summarized as follows: . The Microtox2 90% Comparison Test provided an expedient means of quantifying the bioassay response of waste foundry sand leachates. . Seven out of 11 waste sands generated by ductile and gray iron foundries exhibited bioassay patterns which were either equivalent, or even superior, to levels observed with virgin sands. . Four out of 11 waste sands generated by this same group of ductile and gray iron foundries, however, showed elevated bioassay response levels which suggested that an inhibitory contaminant had been released from these sands. . There appeared to be a correlation between higher levels of bioassay inhibition and leachate soluble COD. . While the exact nature of the contaminants responsible for the higher levels of bioassay inhibition was not ascertained, the latter correlation with sCOD, and rapid (i.e., 5 min) appearance of the inhibition, suggests that these chemicals were organic in form. . This contamination appears to be associated with the use of particular types of core or mold binders.
.
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The three highest levels of bioassay inhibition were, in fact, observed at those foundries using either hot box core (two each) or chemically bound mold (one each) binders. Although no leachate sCOD tests were conducted with the non-iron foundry sands, the fact that they both exhibited elevated bioassay inhibition was believed linked to the reduced levels of core binder burn out which would have been maintained in either of these operations. Although no direct correlation between Microtox2 bioassay behavior and environmental risk has been demonstrated for these waste sands, the comparison of foundry residuals with virgin materials is considered to be defensible in terms of accepting or rejecting these sands for bene®cial reuse. The fact that the majority of ductile iron foundry sands appeared to be `clean' based on their bioassay response re¯ects the historical fact that many of these sands have been used for decades of successful reuse. Signi®cant economic bene®ts have, in turn, been realized by residual generators and end users alike, particularly in terms of their conservation of land®ll space and associated monetary savings. However, while many ferrous foundry residuals have been chemically quali®ed as being acceptable for reuse according to current environmental regulations, underlying concerns about product liability do suggest that this sort of additional bioassay testing would be extremely useful. Based on these results, a fully monitored, 50,000 cubic yard highway embankment in northeast Indiana using Microtox2-quali®ed ductile iron foundry residuals was built during the summer of 1996. Subsequent chemical-speci®c and bioassay testing using surrounding groundwater monitoring wells has shown no indication of any negative environmental impact.
Acknowledgements This research eort was developed, sponsored, and catalyzed by the Indiana Department of Transportation, with complementary ®scal support and assistance by the Indiana Cast Metals Association and its industrial members.
References [1] Casting Industry Suppliers Association, Sand Committee Task Force. The right thing to doÐwith spent sand. Foundry Manage. Technol. 1995;123(3):27±9.
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[2] Foundry Management and Technology. Sand/binders/sand preparation/coremaking. Foundry Manage. Technol. 1993;121(1):D3D12. [3] Partridge BK, Alleman JE, Huber BW. Perspectives on liability for constructive reuse of high volume waste streams. Transportation Research Record 1997;1577:109-17. [4] Boyle WC, Ham RK. Assessment of leaching potential from foundry process solid wastes. Proc. Purdue Ind. Waste Conf. 129;34:1979. [5] Ham RK, Boyle WC, Kunes TP. Leachability of foundry process solid wastes. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1981;107: 155.
[6] Ham RK, Boyle WC, Engro EC, Fero RL. Organic compounds in ferrous foundry process waste leachates. J. Envir. Eng 1993;119:34. [7] Ham RK, Boyle WC, Traeger P, Wellender, D, Lovejoy M, Hippe JM. Evaluation of foundry wastes for use in highway construction. Final Report to Wisconsin Departments of Natural Resources and Transportation, 1993. [8] Stanforth R, Oman D. Bene®cial use of foundry by-products in highway construction project review: a review of research conducted by the University of Wisconsin. Madison (WI): RMT, Inc., 1994 [9] Microbics Corporation. Manual for fresh water and waste samples. Microbics corporation, Carlsbad, CA, 1995.
Microtox2 characterization of foundry sand residuals K.C. Bastian a, J.E. Alleman b* a US Geological Survey,Lawrence, KS 66049-3839 ,USA School of Civil Engineering Purdue University,West Lafayette, IN 47907-1284 USA
b
Received 9 July 1998; received in revised form 8 May 1998; accepted 20 May 1998
Abstract Although foundry residuals, consisting mostly of waste sands, represent a potentially attractive, high-volume resource for bene®cial reuse applications (e.g. highway embankment construction), prospective end users are understandably concerned about unforeseen liabilities stemming from the use of these residuals. This paper, therefore, focuses on the innovative use of a microbial bioassay as a means of developing a characterization of environmental suitability extending beyond the analytical coverage already provided by mandated chemical-speci®c tests (i.e., TCLP, etc.). Microtox2 bioassays were conducted on leachates derived from residuals obtained at a wide range of facilities, including: 11 gray and ductile iron foundries plus one each steel and aluminum foundries. In addition, virgin sand samples were used to establish a relative `natural' benchmark against which the waste foundry sands could then be compared in terms of their apparent quality. These bioassay tests were able to eectively `®ngerprint' those residuals whose bioassay behavior was comparable to that of virgin materials. In fact, the majority of gray and ductile iron foundry residuals tested during this reported study elicited Microtox2 response levels which fell within or below the virgin sand response range, consequently providing another quanti®able layer of support for this industry's claim that their sands are `cleaner than dirt.' However, negative Microtox2 responses beyond that of the virgin sands were observed with a number of foundry samples (i.e. four of the 11 gray or ductile iron sands plus both non-iron sands). Therefore, the latter results would suggest that these latter residuals be excluded from bene®cial reuse for the immediate future, at least until the cause and nature of this negative response has been further identi®ed. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Waste foundry sand; Bioassay; Microtox2; Waste reuse; Environmental quality
1. Introduction Each year, the US foundry industry utilizes over 5 million tons of sand in expendable mold casting operations [1]. In these operations, molten metal is poured into molds made of sand which has been shaped and hardened in conjunction with various additives to withstand the pressure and heat derived from the heated metal. After the metal has cooled, the sand is separated from the casting and foundries then go to great lengths to recycle as much sand as possible. Some wastage after each casting cycle is necessary, though, due to physical and chemical deterioration of the sand and to the fact that virgin sands are required in preparing certain sections of the mold. The vast majority of the wastes is sand, usually silica-based, although some olivine, zircon, chromite, and other base sands are used [2]. However, these wastes also contain residues of various * Corresponding author. Tel.: +1-765-494-7705; fax: +1-765-4961107; e-mail: [email protected]
casting components, including clays, coal and petroleum derivatives, and other organic chemicals. A schematic of foundry processes and material ¯ows is presented in Fig. 1. The constructive reuse of these high-volume foundry residuals represents a decidedly bene®cial goal with distinct economic and environmental bene®ts. Many potential end users, however, are reluctant to use these residuals given an inherent concern about unforeseen liabilities [3]. These wastes have already been extensively tested in compliance with current regulations regarding leachate characterization (i.e. using leachate generation and testing protocols such as the TCLP), and the available results strongly suggest that many gray and ductile iron foundries are discarding residuals whose quality is fully amenable to their future use with a variety of bene®cial reuse applications such as highway embankment construction [4±7]. In fact, given the preponderance of data showing low contaminant leachate levels, these wastes have been optimistically quali®ed by industry spokesmen as having less potential impact on ground-
0956-053X/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved PII: S0956 -0 53X(98)00030-0
228
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Fig. 1. Schematic of foundry processes and material ¯ows.
water than do virgin soils [8]. In spite of this industry's data and their related claims about `cleanliness,' though, lingering uncertainties still persist regarding the longterm behavior of these materials in terms of their potential release of chemical contaminants not covered by current testing. As such, these materials are commonly land®lled at considerable expense to the responsible foundries, while at the same time unduly consuming precious land®ll volume. This project was consequently developed to provide a complementary assessment protocol based on surrogate biological indicators which could be used to eectively `®ngerprint' those residuals whose bioassay behavior was comparable to that of virgin materials. Quite frankly, foundry waste sands can be exceedingly complex in terms of the potential composition and character of their leachates. The dominant concern largely revolves around organic residues found within the waste sand, stemming from the original use of complex sand binders or, perhaps, from byproducts created from these latter compounds during the casting process. These `synthetic' additives, used in conjunction with a diverse group of `natural' materials (including clays, sea coal, and wood derivatives) provide the requisite surface and strength properties for the involved mold and core casting components. In many instances, the casting temperatures associated with ferrous metals are suciently high to thoroughly `burn out' these mold and core organics. However, with various types of casting
techniques and/or binding additives, it would appear that some of the waste sands still contain a diverse range of undesirable organic contaminants. Mandating chemical-speci®c testing for each of these potential contaminant species would be exorbitantly expensive and, even then, these individual tests would provide no information about potential complications caused by problematic synergisms. Leachate bioassay testing, though, would provide an all-encompassing characterization of this material's potential to impose a negative environmental impact, thereby generating an exceedingly useful assessment of a waste's fundamental quality and suitability as compared to, and judged against, virgin sands. Extending beyond chemical-speci®c leachate testing (e.g. TCLP), our research group consequently developed a foundry residual leachate generation and testing protocol based on the Microtox2 bioassay system. This test uses light producing bacteria to characterize the presence of inhibiting contaminants, as measured by the resultant suppression of their light-emitting activity. In turn, these results provided a biological assessment of foundry wastes in terms of their environmental character. Although the commercial name `Microtox2' inherently implies that `toxicity' is being evaluated, the relative meaning of any such `toxic' indication is subject to considerable interpretation. All of the tested residuals during this project had, in fact, already been proven to be `non-toxic' (by a considerable margin) according to
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the applicable regulatory criteria. The involved concept of using a Microtox2 test to evaluate these foundry residual leachates, therefore, is moreso focused on qualifying a bioassay response relative to virgin materials. For those samples which fell within the response range established for clean, natural sands, their Microtox2 behavior provided a strong indication that these residuals are suitable for subsequent reuse. Conversely, for those residuals whose Microtox2 response showed excessive negative impacts (i.e. decreased light emission as compared to virgin sands), these results suggest that their exclusion from further consideration for bene®cial reuse would be warranted until the causative factors can be established. This relatively simple and inexpensive test was correspondingly designed to evaluate each waste residual in comparison to the relative bioassay behavior of virgin materials. As documented within this report, the `cleanliness' of the residuals obtained from many foundries was readily evident. Leachates from seven of the iron foundries tested caused less inhibition of light production by the Microtox2 bacteria than did virgin sands. Taken literally, it appears that these residuals are, truly, `cleaner than dirt.' Furthermore, for these wastes, no real dierences were seen between system sands and fresh or aged waste sands. However, in a limited number of instances, there were clear and consistent indications that the tested foundry wastes had released an inhibitory contaminant into the leachate waters, thereby resulting in a quanti®able depression in observed microbial activity. For four iron foundries, the Microtox2 responses to fresh waste sand leachates were noticeably less positive than to system sands, and these responses generally varied more than those elicited by residuals from the seven `clean' foundries. Residuals from the steel foundry and the aluminum foundry also displayed inhibition of light production which was consistently greater than that caused by virgin sand leachates. Overall, therefore, this innovative bioassay test appears to oer an ecient and expedient approach to `®ngerprinting' foundry locations for which constructive waste sand reuse could subsequently be pursued without undue concern about negative environmental impacts. 2. Materials and methods 2.1. Sample procurement One-half- to 2 kg samples of system and fresh waste sand were collected by foundry or university personnel from a total of 13 foundries for testing. These included 10 iron greensand (e.g., using primarily clay-based binders) foundries, one iron foundry which used organic chemicals as the primary binders, one steel greensand foundry, and one aluminum greensand foundry. In
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addition, aged waste sand samples were obtained from two of the iron greensand foundries, the steel foundry, and the aluminum foundry. Between six and 37 samples were obtained from each facility over periods ranging from 5 days to 6 months. Finally, 13 virgin sand samples, donated by foundries or sand manufacturing companies, were also subjected to bioassay testing. The majority of these samples were packed in glass jars, cans, or zipper storage bags and returned to Purdue University for storage at 4 C prior to eventual testing. All samples were labeled with the appropriate information to identify sample location, type, and date. 2.2. Leachate generation From each stored sample, 20.000.05 was taken and placed in a ¯ask containing 801 ml of a 2% NaCl solution. As compared to the relative level of solids used in the TCLP test, therefore, the solids to liquid ratio used during these leachate studies (i.e. 20:80) was fourfold higher (i.e., thereby being more conservative ). During the development of various testing regimes with the Microtox2 90% Comparison Test (see following discussion), the use of a saline leachant was considered necessary in those cases where the bacteria were actually added directly to the leachate suspensions (to simulate the natural, marine environment of the Microtox2 organisms). Although the ®nal leaching strategy did not include these bacteria, this saline leachant protocol was continued as yet another conservative measure, by way of prospectively enhancing the release of bound contaminants (e.g. metals released by way of ion exchange, etc.). Each ¯ask was covered with para®lm, manually agitated to break up clumps, and placed on a shake table for 182 h. The solution was then allowed to settle and 30 to 40 ml of supernatant was poured into polycarbonate centrifuge tubes. The samples were centrifuged for 16 min at 10,000 rpm (approximately 10,000 g). The centrifuge supernatants were ®ltered through 1.5 mm pore size glass ®ber ®lters. The pH was then recorded and, when necessary, a phosphate buer solution was used to adjust pH to a level between 6.5 and 8.0. Samples were analyzed immediately or transferred into borosilicate glass vials, covered with parafilm, capped, and stored at 4 C for no more than 72 h. The control tests were conducted with the same NaCl solution, but without any sand. 2.3. Microtox2 testing protocol The Microtox2 90% Comparison Test used during these tests to determine foundry sand toxicity was recommended and developed by the involved commercial vendor speci®cally for samples believed to contain low levels of contaminant toxicity [9]. As mentioned
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previously, this test uses a special light emitting bacterial (Vibrio ®scheri) suspension whose energy emission in the presence of unknown samples can then be quanti®ed within a darkened chamber using a photodetector. The reported results derived with this test subsequently represent the percentile decrease in light emission measured between the control (®ve each) and sample (®ve each) cuvettes. Short- and long-term testing protocols were initially devised for this test, and in some instances the bacteria were actually added to the shaken leachate suspensions. However, the results presented in this paper were all obtained using a ®nal testing strategy in which the bacteria were added after the multi-hour leaching step. In each test, 5 leachate replicates (i.e. derived from 182 h of contact with the tested samples) were compared with ®ve control replicates consisting of unamended 2% NaCl solution. The actual process of conducting any given test can be summarized as follows. The freezedried Microtox2 bacterial culture was initially reconstituted and, in each of ten cuvettes, the light produced by 0.1 ml of this bacterial suspension was measured. A 0.9 ml sample was then added to ®ve cuvettes and 0.9 ml control diluent was added to the other ®ve cuvettes. Light output from each cuvette after 5 and 15 min was measured, and the percent dierence between initial light output and ®nal light output was quanti®ed. The replicate values were averaged and an overall toxicity value and range of certainty were calculated. For quality control purposes, complementary bioassay tests were conducted on an approximately monthly basis using reference toxicants. A known organic toxicant (10 mg/l phenol) was used for this purpose, thereby verifying the repetitive sensitivity of the employed Microtox2 cultures. 2.4. Additional leachate testing In addition to routine measurements of the their pH levels, representative leachates from each of the tested foundries were also evaluated in terms of their resultant chemical oxygen demand (COD) concentrations (using HACH dichromate digestion tubes). The latter COD tests were conducted on ®ltered leachates, such that they represent soluble concentrations (i.e. sCOD).
important to remember that the Microtox2 response data is not being used to de®nitively calibrate toxicity, but to identify and `®ngerprint' those residuals whose quanti®able character appears commensurate with that of natural materials (i.e. perhaps not ``cleaner than dirt,'' but eectively indistinguishable)! 3.2. pH Virgin sand pH values averaged approximately 6.9, being slightly acidic. Iron greensand foundry residual leachates were usually slightly basic, although pH levels ranged from 6.5 to 10.1. The chemically bound waste foundry residual leachates had neutral pH levels, ranging from 6.8 to 7.3. Leachates from aluminum foundry waste residual leachates were slightly basic. Steel foundry waste residual leachate pH levels were not measured. No apparent correlation existed between initial pH and bioassay response. 3.3. Microtox2 Fig. 2 exhibits Microtox2 5-min response data for the virgin sand samples. In this ®gure, each data point represents the response of one sample (i.e. percentile values indicating the loss of light emission between the control and sample tests), with vertical error bars representing the estimated error of each measurement. 15-minute response data closely paralleled the data shown. For the 13 samples, mean inhibition of light production by virgin sand leachates was 9.0 and 9.2% for 5- and 15-minute readings, respectively, with standard deviations of 7.7 and 6.9%. A solid horizontal line is used to represent the mean responses, while dotted horizontal lines indicate one standard deviation above and below mean values. As response to virgin sands is the standard to which responses to foundry residuals will be compared, similar horizontal lines indicating mean and standard deviation of responses to virgin sands were also included in all ®gures displaying bioassay data.
3. Results and discussion 3.1. Overview Data were obtained for 13 virgin sand samples from various sources and for samples from eleven gray and ductile iron foundries, one steel foundry, and one aluminum foundry. Measurements included leachate pH values and 5- and 15-min Microtox2 response data. It is
Fig. 2. Virgin sand MicrotoxTM (5-min) response.
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Again, these latter percentile response levels are relative to a `clean' control water whose known composition would not have been expected to exert a negative impact on the employed bacteria. By comparison, the virgin sand leachates engendered a range of bioassay eects which on average were marginally `inhibitory' (i.e. with higher percentile dierences in light production for the control versus sample tests) due to an unknown stress. While the magnitude of this impact was not at all extreme, there was no readily obvious reason why the virgin sands should behave in this fashion. The fact remains, though, that these virgin sand results qualify as the best natural benchmark against which the foundry residuals can directly be judged. Fig. 3 displays a summary of the 5-min Microtox2 response levels observed with residuals from all 13 foundry facilities. Here again, these bioassay results (depicted on the y-axis) are quanti®ed as a percentile decrease in light output for test microorganisms exposed to sand leachates as compared to those exposed solely to the specially prepared `control' water. Each data point depicts the mean response for all samples from the waste type it respectively represents. The range of response values from each sample type is shown by the vertical bars passing through the data points. Types of data include system sands (e.g. sands which have been prepared to be used as mold sands), fresh waste sands, aged waste sands, and air handler dusts (e.g. ®nes resulting from the physical breakdown of mold sands). It is seen that the Microtox2 response varied greatly. The bioassay response by residuals from 11 of eleven iron foundries (F1 through F7) was consistently below the average response to virgin sands. Residuals from three of the iron foundries (F8 through F10) produced
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inconsistent responses. Sands generated by the single foundry (F11) which utilized a synthetic organic mold sand binder, rather than a clay-based `greensand' process, displayed a consistently inhibitory eect on the Microtox2 organisms. The responses to fresh waste sands from the steel foundry (N1) and all residuals from the aluminum foundry (N2) demonstrated signi®cant bacterial inhibition. Only one aged waste sand sample from the steel foundry was available; Microtox2 response to leachate from this sample was below the virgin sand average response. An example of data from ``clean'' residuals is shown in Fig. 4. Microtox2 responses (5- and 15-min) to individual fresh waste sand samples taken from this foundry (Foundry F3) over a period of 6 months are shown. In this ®gure (as in the remaining Microtox2 response ®gures), each data point represents the response of one sample, with error bars representing the estimated error of each measurement. Two points are worthy of note. First, no clear pattern of increase or decrease in inhibition of light production is seen over the sampling period. Second, the 15-minute Microtox2 responses are not signi®cantly greater than the 5-min responses. As V. ®scheri response to metals is not generally observed within 5 min, this latter observation suggests that organics, rather than metals, are causing the bacterial inhibition. As suspected, therefore, chemical-speci®c testing which focuses on metals or other inorganics (as does the presently mandated characterization) may not provide an eective estimate of potential environmental impact. Indeed, Microtox2 testing provided no indications of metals-related inhibition for any of the residuals tested. While the response to Foundry F3 fresh waste sands was consistently within the range established by virgin
Fig. 3. MicrotoxTM response to foundry residuals after 5 min.
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sand testing, samples from other greensand foundries evoked wide variations in Microtox2 response (see, for example, Fig. 5). For this foundry (F10) (and that of the inhibition seen with Foundry F9 as well), it is probable that the bacterial inhibition was caused by the particular core binder chemicals, including so-called `hot box' binders, which were used. The reason(s) for the dierences in inhibition over a period as little as 1 week, however, cannot be determined without additional testing; possibilities include variations in sampling procedures or in casting processes. A consistently high level of bacterial inhibition was observed with samples from the foundry which used an organic chemical (rather than greensand) binder system for its mold sands (Foundry F11; see Fig. 6). The samples from this foundry had a dierent color than most of the other residual samples and had a distinct organic chemical odor, likely due to binder chemicals remaining on the residuals after casting. One major dierence between iron and steel casting is casting temperature (1565 C for steel, as opposed to approximately 1370 C for iron). It might be hypothe-
sized that any contamination due to organics would be decreased, due to the higher temperature. Surprisingly, however, the inhibition of light production by Microtox2 organisms exposed to steel foundry fresh waste sand leachates was signi®cantly greater than the bacterial response to virgin sand leachates (see Fig. 3). This behavior could stem from the use of particular binders or from the use of greater quantities of applied binder chemicals than are used for iron casting. Alternatively, due to the large casting sizes produced by this foundry (up to 4000 kg), the cores may have been of such large size that inner core temperatures were not high enough to oxidize all binder chemicals. Aluminum is cast at a much lower temperature than iron (approximately 700 C). Consequently, the molds and cores break down to a much smaller extent during casting, necessitating the use of additional vibration or shaking to separate the sand from the castings after the metal has cooled. Much higher levels of organic contamination of aluminum foundry residuals were expected, and the results of Microtox2 testing were consistent with such expectations: bacterial inhibition levels by
Fig. 5. Foundry F10 fresh waste sand MicrotoxTM (5-min) response.
Fig. 4. Foundry F3 fresh waste sand MicrotoxTM response.
Fig. 6. Foundry F11 fresh waste sand MicrotoxTM (5-min) response.
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system, fresh waste, and even aged waste sand leachates were consistently higher than had been seen in iron or steel foundry sands (see Fig. 3). 3.4. COD There was a sizable variation in the levels of sCODs measured within the waste foundry sand leachates derived during this study. At the low end of this spectrum, the virgin sands had the lowest soluble COD values, with a range of about 2±24 mg/l. Furthermore, the foundries which exhibited the best (i.e. least inhibitory) levels of Microtox2 response also had the lowest observed sCOD values. Foundries F1 and F2 samples had respective sCOD ranges of 25±28 and 50±65 mg/l. At the other extreme, there did appear to be a correlation between higher levels of Microtox2 response and leachate sCOD. For example, Foundry F9 had a sCOD range of 125 to 160 mg/l, and inhibition levels consistently above the virgin sand benchmark. Similarly, Foundry F11 had an sCOD range of 70±130 mg/l, and an inhibitory percentile of about 60%. Lastly, Foundry 10 had the widest range of observed inhibition with its fresh and aged sands, and here again the corresponding sCODs also had the highest level of observed variation (ranging from 25 to 210 mg/l).
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4. Summary The ®ndings and corresponding observations derived from this study can be summarized as follows: . The Microtox2 90% Comparison Test provided an expedient means of quantifying the bioassay response of waste foundry sand leachates. . Seven out of 11 waste sands generated by ductile and gray iron foundries exhibited bioassay patterns which were either equivalent, or even superior, to levels observed with virgin sands. . Four out of 11 waste sands generated by this same group of ductile and gray iron foundries, however, showed elevated bioassay response levels which suggested that an inhibitory contaminant had been released from these sands. . There appeared to be a correlation between higher levels of bioassay inhibition and leachate soluble COD. . While the exact nature of the contaminants responsible for the higher levels of bioassay inhibition was not ascertained, the latter correlation with sCOD, and rapid (i.e., 5 min) appearance of the inhibition, suggests that these chemicals were organic in form. . This contamination appears to be associated with the use of particular types of core or mold binders.
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The three highest levels of bioassay inhibition were, in fact, observed at those foundries using either hot box core (two each) or chemically bound mold (one each) binders. Although no leachate sCOD tests were conducted with the non-iron foundry sands, the fact that they both exhibited elevated bioassay inhibition was believed linked to the reduced levels of core binder burn out which would have been maintained in either of these operations. Although no direct correlation between Microtox2 bioassay behavior and environmental risk has been demonstrated for these waste sands, the comparison of foundry residuals with virgin materials is considered to be defensible in terms of accepting or rejecting these sands for bene®cial reuse. The fact that the majority of ductile iron foundry sands appeared to be `clean' based on their bioassay response re¯ects the historical fact that many of these sands have been used for decades of successful reuse. Signi®cant economic bene®ts have, in turn, been realized by residual generators and end users alike, particularly in terms of their conservation of land®ll space and associated monetary savings. However, while many ferrous foundry residuals have been chemically quali®ed as being acceptable for reuse according to current environmental regulations, underlying concerns about product liability do suggest that this sort of additional bioassay testing would be extremely useful. Based on these results, a fully monitored, 50,000 cubic yard highway embankment in northeast Indiana using Microtox2-quali®ed ductile iron foundry residuals was built during the summer of 1996. Subsequent chemical-speci®c and bioassay testing using surrounding groundwater monitoring wells has shown no indication of any negative environmental impact.
Acknowledgements This research eort was developed, sponsored, and catalyzed by the Indiana Department of Transportation, with complementary ®scal support and assistance by the Indiana Cast Metals Association and its industrial members.
References [1] Casting Industry Suppliers Association, Sand Committee Task Force. The right thing to doÐwith spent sand. Foundry Manage. Technol. 1995;123(3):27±9.
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[2] Foundry Management and Technology. Sand/binders/sand preparation/coremaking. Foundry Manage. Technol. 1993;121(1):D3D12. [3] Partridge BK, Alleman JE, Huber BW. Perspectives on liability for constructive reuse of high volume waste streams. Transportation Research Record 1997;1577:109-17. [4] Boyle WC, Ham RK. Assessment of leaching potential from foundry process solid wastes. Proc. Purdue Ind. Waste Conf. 129;34:1979. [5] Ham RK, Boyle WC, Kunes TP. Leachability of foundry process solid wastes. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1981;107: 155.
[6] Ham RK, Boyle WC, Engro EC, Fero RL. Organic compounds in ferrous foundry process waste leachates. J. Envir. Eng 1993;119:34. [7] Ham RK, Boyle WC, Traeger P, Wellender, D, Lovejoy M, Hippe JM. Evaluation of foundry wastes for use in highway construction. Final Report to Wisconsin Departments of Natural Resources and Transportation, 1993. [8] Stanforth R, Oman D. Bene®cial use of foundry by-products in highway construction project review: a review of research conducted by the University of Wisconsin. Madison (WI): RMT, Inc., 1994 [9] Microbics Corporation. Manual for fresh water and waste samples. Microbics corporation, Carlsbad, CA, 1995.