CHROME RECOVERY VIA ADSORPTIVE FILTRATI ON. Lisa M. Brown U.S. Environmental Protection Agency Risk Reduction Engineering Laboratory

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1 CHROME RECOVERY VA ADSORPTVE FLTRAT ON Lisa M. Brown U.S. Environmental Protection Agency Risk Reduction Engineering Laboratory 26 W. Martin Luther King Cincinnati, Ohio Mark M. Benjamin and Thomas Bennett Department of Civil Engineering FX-10 University of Washington Seattle, Washington ABSTRACT Metal-bearing wastewaters are generated in large quantities at thousands of industrial sites in the U.S. The concentrations of metals in these waters may range from less than 1 to several hundred mg/l. The vast majority of these waste streams are treated by processes which generate a large amount of metalcontaining sludge which then must be de-watered, transported, and buried in a controlled landfill. The U.S. Environmental Protection Agency (USEPA), through the Office of Research and Development, has a national research program designed to support the intent of the 1984 Amendments to the Resource Conservation and Recovery Act of reducing the amount of hazardous and non-hazardous waste produced in the United States. This research program focuses on generation of data to allow the development of emerging new pollution prevention techniques. The University of Washington, under a thirty-one month cooperative agreement with the USEPA, is evaluating the performance of packed beds of granular media coated with iron oxide and other adsorbents for recovering chromate from industrial waste solutions. The initial tests are being conducted using synthetic wastes. Following that, tests will be conducted using batches of real waste. A small recovery unit will be installed on-site at an industry near the University at the culmination of the project for pilot-scale evaluation. This paper has been reviewed in accordance with the U.S. Environmental Protection Agency's peer and administrative review policies and approved for presentation and publication. 100

2 effective waste reduction and recycling programs effort, an assessment will be performed for the selected Air Force base. The project is being p with Auburn University. Objectives for the stud mitigation technology for minimizing the accumul product and addressing concerns additives. The results of the st program and establ i sh protocol ogy service programs. \ p SUMMARY AND CONCLUS S tion opportunity asse ts conducted under EPA s an excel 1 ent mechani s promoting pollution e three DOD sites audited, n identified for the following hazardous waste ms: aluminum ng solutions, paint sludge, paint solutions, waste battery acid, parts washer wastew ng, parts cleaning solutions, waste Otto fuel, an Pollution preventi Philadelphia Naval Ship endations have been made for the Fort Riley that will reduce hazardous waste pounds per year, with an n addition, a number of excel 1 ent demonstration and eval uat i on as a catalyst to implement pollution research opportunities that would not s program EPA can assist DOD in of its facilities through \ \ 99

3 NTRODUCTON Recent work at the University of Washington has led to the development of a new material which may allow metals to be efficiently and cost-effectively recovered from metal-bearing wastewater. The material consists of either ordinary filter sand or activated carbon which has been coated with a thin layer of an adsorbent mineral such as iron oxide. When wastewater is adjusted to an appropriate ph and passed through a column packed with this material, the oxide coating adsorbs the soluble metals and filters out the particulate matter. Once the columnls capacity has been exceeded, the material can be regenerated simply by passing another solution through at a different ph. The regenerant solution typically contains the metal at a concentration about 100 or more times that of the water which was treated. This process eliminates a major constituent which is generated during conventional treatment of metal-bearing wastewaters (iron or aluminum hydroxide) and generates a product stream from which the waste metals may be recycled or recovered, thereby avoiding both the cost and risks associated with dispgsal. n October of 1989 the U.S. Environmental Protection Agency (EPA), Risk Reduction Engineering Laboratory began a thirty-one month cooperative agreement with the University of Washington to develop this technology as a recycling option for use at metal finishing facilities. Chrome containing wastewaters were selected as the target streams due to the toxicity and the volume generated. The objective of this project is to evaluate the performance of packed beds of granular media coated with iron oxide and other adsorbents for recovering chromate from industrial waste solutions. The initial tests are being conducted using synthetic wastes. Following that, tests will be conducted using batches of real waste. A small recovery unit will be installed on-site at an industry near the University at the culmination of the project for pilot-scale evaluation. The experimental tasks have been divided into three phases: (1) optimization of the process for coating the media with an adsorbent surface examining a) coating efficiency and b) dissolution of coating; (2) optimizing collection and recovery of chromate from relatively dilute synthetic waste solutions; and, (3) testing the process with real industrial wastes both at bench-scale and on-line at an industrial site. DESCRPTON OF THE TECHNOLOGY Conventional technology for removing metals from solution involves precipitation of cationic metals as oxides, hydroxides, or sulfides, and then separating the particulate metals by settling, usually in conjunction with a coagulant such as iron 10 1

4 hydroxide. This approach has several practical limitations, some of which are exacerbated when the metals are present in complex matrices such as many industrial wastewaters. Among these are that precipitation is ineffective for most anionic metals (e.g. CrO,, SeO,, VO,) and for metals which are present as inorganic or organic complexes, and that those metals which do precipitate usually form small particles which do not settle readily. This means that large settling basins are typically required to collect the metals, usually followed by a sludge thickening operation. To ensure good treatment efficiency and good coagulation, massive amounts of iron salts are often added, significantly increasing the sludge generated in the process. When ferrihydrite coated sand is placed in a column and a solution containing metals is passed through it, the coating can adsorb the dissolved metals rapidly and efficiently. At the same time, the column acts as a normal granular media sand filter to remove precipitated metals and other particulate matter. After some period of time, either the coating reaches its maximum capacity to remove metals or the filter requires backwashing. this time, a ph-adjusted backwashing solution can be applied to rekover the metals and regenerate the column for further use. Because the ferrihydrite is trapped on the sand particles, only the contaminant metals, not the ferrihydrite, are released. Since the sand and the associated ferrihydrite are retained in the column at all times a settling step is not required. This means the technology requires relatively small amounts of space and treatment chemicals. The adsorptive filtration process generates a concentrated regenerant solution. This regenerant may be recyclable in the production process or have economic value to other users. n an industrial waste stream dominated by one metal, e.g. a segregated Cr(V1) waste stream in a plating shop, it may be possible to simply recycle the regenerant solution to the process. At NVESTGATON OF THE COATNG EFFCENCY OF RON ONTO SAND nitial experiments were conducted to characterize the media prior to its exposure to any waste solutions. During work on optimizing the coating process, the amount of iron coated onto the sand, the efficiency of the coating process and the durability of the coating when the media is exposed to various physical and chemical stresses were evaluated. To determine the amount of iron which is typically coated onto the sand from various solutions, the sand was subjected to 10 coating cycles with each solution tested, and the concentration of Fe on the sand was analyzed after each cycle. Briefly, a cycle consisted of pouring the solution over the sand and heating the mixture at 11OC overnight, at which time the sand appeared to be completely dry. At this point, the sand was weakly "cemented" together. The individual grains were 102

5 separated by gentle grinding using a mortar and pestle. The grains were then rinsed for 15 minutes by placing them in a 200- ml graduated cylinder and flushing de-ionized water through the cylinder from the bottom at a rate sufficient to expand the media by approximately 25%. After 15 minutes, the flow rate was briefly increased to expand the bed to between 75 and 100% of its packed volume to rinse out any small particles which remained. The water flow was then stopped, a small sample of sand was taken, and new Fe-containing solution was added to start the next coating cycle. Sub-samples of coated sand from each cycle were placed in hot, concentrated nitric acid to dissolve the iron coating, and the resulting solutions were analyzed to determine the amount of attached iron. The iron-containing solutions used in the coating experiments varied in terms of the associated anion (nitrate or chloride) and the amount of base added prior to the heating step. The concentration of iron in the coating solution was 1.45 M for the solutions made with iron chloride, and 1.30 M in those made with iron nitrate. Forty ml of iron-containing solution was applied to 150 grams of sand in each cycle, unless otherwise indicated. This amount of solution was just enough to cover the sand. Figures 1 through 3 show the amounts of Fe attached to the sand after each coating cycle. Figure 1 shows the Fe attachment trend for two duplicate batches of sand coated using a solution of iron nitrate containing no added base, and for one sample using twice the volume of coating solution (80 ml of 1.30 M Fe per 150 g sand) per cycle as in the others. The results show good reproducibility with respect to coating efficiency. The amount of iron attached increases after most cycles. Although there are occasional cycles where the increment in attached Fe is zero or even negative, the overall trend is toward steadily increasing attached Fe concentrations, reaching 6.3% and 6.4% Fe by weight, respectively in the duplicate samples, and 11.1% in the sample using a double dose of coating solution. There was no indication that these values were approaching a plateau. Figures 2 and 3 show the comparable data for coating solutions prepared with iron (111) chloride, rather than iron nitrate. n this case, when no base was added, some iron attached for a few cycles, but then was released from the surface in subsequent cycles. The amount of iron on the surface never exceeded 5% by weight of the sand, and at the end of 10 cycles the attached Fe was 3.0% and 3.7% by weight in duplicate samples. Next, samples were prepared in which various amounts of based (10 M NaOH) were added to the coating solution prior to the heating step. The amount of base to add was chosen as follows. Base was added to the coating solution until a precipitate formed which did not dissolve when the solution was stirred and heated slightly. This occurred when the base addition was 0.38 mol NaOH per L of solution. The coating solution contained 1.45 mol Fe/L, 103

6 so the base addition when the non-dissolving precipitate formed was equivalent to mol OH/mol Fe. This was chosen as the maximum amount of base to add to any solution, and solutions were also prepared containing 25, 50, and 75% of this amount of base. As Figures 2 and 3 indicate, the amount of Fe attaching to the sand was not increased significantly by the addition of 0.05 or 0.10 mol OH/mol Fe. As occurred when no base was added, the amount of Fe attached increased and decreased in an apparently random fashion, never exceeding a few weight percent. When the base addition was increased to 0.15 mol OH/mol Fe, the amount of Fe attached still fluctuated somewhat, but reached highe.r levels than in the samples with less base addition. However, when the full 0.21 mol OH/mol Fe was added, the amount of Fe coated increased quite steadily, reaching a value of 8.1% Fe by weight after 10 cycles. As with the coating cycles using an Fe(NO), salt, there was no indication that the amount of Fe coated Lad reached a plateau. ACD DSSOLUTON OF COATED RON The next set of experiments investigated the stability of the coating when exposed to acid solutions. n a full-scale operation, such exposure would occur either during acid regeneration of columns used to remove cationic contaminants (e.g. most metals) or during the treatment cycle of anions (e.g. chromate). n the former case, the ph of the acid solution would probably be around 2.0, with exposure periods of less than 1 up to about 2 hours per regeneration cycle. n the latter case, the exposure period would be much longer but the ph would probably not be as low. A ph value of 2.5 to 3.0 would probably be used in these cases. For the acid exposure tests, a worst-case scenario was simulated, representing treatment to remove chromate at ph 2.0. The test protocol involved exposure of the media to a ph 2.0 solution for 20 hours, followed by 2 hours of exposure to a solution containing 1 M NaOH (approximately ph 14), followed by 2 hours without flow, in contact with the 1 M NaOH. During the periods with water flow, the flow rate was 10.9 ml per minute through a column packed with 21.8 ml of media, corresponding to an empty bed hydraulic detention time of 2 minutes. During each daily test, some fine particles appeared to settle atop the media. These were washed out by a very brief (<1 minute) backwashing step using the 1 M NaOH prior to returning to the ph 2 flow tests. Two acid dissolution tests under these conditions have been cgmpleted. n these tests, the media used were those prepared with FeC13 and either the maximum base addition or 50% of this amount of base. cycles. hours. Both sets of media had been coated in 10 coating The tests lasted 14 days, with samples collected every 2 After 7 days and again at the end of the 14-day test, a 104

7 sub-sample of the media was collected, and all the iron was dissolved off the grains. Analysis of the resulting solution provided a check on the estimates of Fe lost during the test up to that point. The results from these tests are presented in Figures 4 and 5. The amount of iron released from the surface into the acidic water was typically between 0 and 1 mg Fe/L. The release pattern during a 24-hour period usually included a spike immediately after acid flushing began, followed by a gradual decline in Fe released for the remainder of the 22-hour exposure period. The amount of Fe released was consistently less in the sample which had been coated with the more neutralized (greater base addition) solution, particularly with respect to the spike at the beginning of each acid exposure period. Both because of the higher release rate and the lower initial coverage, the media prepared using the less-neutralized solution lost a significantly larger fraction of the total attached Fe than did the more highly-neutralized media (Figure 6). n this test using more highly-neutralized media, ' the daily Fe loss was limited to a few tenths of 1% of the attacbed Fe during the first 12 days of the test and decreased to only 0.1% during the last 2 days. The amount of Fe detected in the acid dissolution effluents was considerably less than the total loss of iron from the media during the 2-week test period. The additional losses were apparently due to the backwashing of the fines during the brief backwashing step each day. These losses were significant during the first week, but appeared to be less or negligible during the latter parts of the test. For instance, although the total Fe loss during the 2-week period for the more- and less-neutralized samples was about 33% and 50%, respectively, in both cases over 90% of the 2-week Fe loss occurred during the first week. This suggests that even if a sample loses a good deal of its attached Fe in the early stages+of its use, the remaining Fe is firmly attached and will not be easily lost. Similar tests with the media prepared using an iron nitrate solution are currently being conducted. CHROMATE ADSORPTON TESTS n the actual adsorption/recovery tests, waste solutions will be passed through columns packed with the coated media. A typical column will contain about 90 ml of bulk media. n order to characterize adsorption of Cr(V1) on to the coated media, Cr(V1) concentration, solution ph, and residence time in the columns will be measured. Three Cr(V1) concentrations will be tested: 10, 75, and 500 mg/l. n the initial tests, the solutions will contain only deionized water and sodium chromate at the appropriate concentration. t is anticipated that tests will be conducted with each solution at 3 influent ph values. At each ph value 105

8 tested, the test run will continue until the ratio of effluent to influent Cr(V1) concentration is at least 0.5. At this point the columns will be regenerated by exposure to a high-ph solution, as described below. At each ph value, at least one run will be conducted at each of 3 flow rates, yielding empty-bed hydraulic residence times of approximately 2, 5, and 15 minutes. f the capacity of the column for adsorbing Cr(V1) appears to be sensitive to flow rate in these tests, additional tests at other flow rates will be conducted. Based on the results of these tests with Cr(V1) only, operational parameters will be chosen for subsequent tests in which competing ionic adsorbates are present. Tests will also be conducted using sulfate and Cr(1) as competing adsorbates. For these tests, only the ph and flow determined as optimal from the tests containing only Cr(V1) will be used initially. f the capacity of the media to collect CR(V1) from the solutions is significantly diminished in these tests compared to those with no competing ions, lower flow rates and different ph values will be tested in an attempt to improve performance. The maximum chromate concentration attainable in the regenerant solution is important because it may ultimately control whether the regenerant solution has economic value (can be recycled). Based on results from previous tests, a single regeneration procedure will be chosen and used for the runs containing competing ions. f the regeneration efficiency in these runs is substantially worse than in those with no competing ions, the regeneration ph and/or flow rate will be changed in an effort to improve regeneration performance. TESTS USNG REAL NDUSTRAL WASTEWATER Chromate-containing wastewater from a local industry will be identified and treated using adsorptive filtration. The initial test runs will use operating parameters identified in the previous tests. Modifications will be made as appropriate to improve process performance, particularly if the process does not perform as efficiently as it did in the tests with synthetic wastes. Once the process has been operated successfully with the industrial wastewater at bench-scale, a pilot unit capable of treating approximately 1 gpm of the wastewater will be constructed and tested on-site. This unit will be subject to any changes in waste composition which occur in the real system, and therefore will provide valuable information about the stability of the system under transient loading conditions. 106

9 GENERAL REFERENCES 1. Benjamin, M.M., "Adsorption and Surface Precipitation of Metals on Amorphous ron Oxyhydroxide,l# Environmental Science Technology, 17, 686 (1983). 2. Edwards, M., and Benjamin, M.M., Regeneration and Reuse of ron Hydroxide Adsorbents in the Treatment of Metal-Bearing J. Water Pollution Control Federation, 61, 481 (1989). 3. Edwards, M., and Benjamin, M.M., Adsorptive Filtration Using Coated Sand: A New Approach for Treatment of Metal- Bearing Wastes.1f J. Water Pollution Control Federation, 61, 1523 (1989). 107

10 Figure 1. Weight fraction of attached iron using a ferric nitrate solution. % F e b Y W E G H T ' COATNG CYCLE - 80 mi coating soln 40 mi coating soln mi coating soln Figure 2. Weight fraction of attached iron using a ferric chloride solution. 6. % F COATNG CYCLE - 40 mi un-neutralized + 40 ml un-neutralized ml M NaOH 108

11 Figure 3. Weight fraction of attached iron using a ferric chloride solution 10 YO F e b Y w E G H T COATNG CYCLE M NaOH M NaOH M NaOH Figure 4. ron Concentration in Effluent 1.45 M ferric chloride M NaOH 2.5 EFFLUENT Fe (ppm) SAMPLE NUMBER 109

12 Figure 5. ron Concentration in Effluent 1.30 M ferric chloride M NaOH 6 EFFLUENT Fe (ppm) SAMPLE NUMBER Figure 6. Loss of attached iron due to % exposure to acid (ph 2.0) solution. F e L s T F R 0 M A C D W A S H N G 0' DAY M NaOH coating -t M NaOH coating 110

13 United States office of Research and EPA'GOO Environmental Protectiori Agency Deb cloprnent Washington DC September 1990 The Environmental Challenge of the 1990's. Proceedings 2 nternational Conference" on Pollution Prevention: Clean Technologies and Clean ProdOcts June 10-13, 1990 "., A*- Printed on Recycled Paper