COMRINED TREATMENT OF HEXAVALENT CHROMIUM WITH OTHER HEAVY METALS AT ALKALINE ph. THOMAS E. HIGGINS RRTAN R. MARSHALL CHZM HILL Reston, Virginia

Transcription

1 COMRINED TREATMENT OF HEXAVALENT CHROMIUM WITH OTHER HEAVY METALS AT ALKALINE ph THOMAS E. HIGGINS RRTAN R. MARSHALL CHZM HILL Reston, Virginia ZNTRODU CT I ON With the introduction of additional and more stringent pretreatment regulations, industries are often finding it diffjcult to meet their discharge requirements. Particularly hard hit are small facjlities with wastes containing low concentrations of hexavalent chromium. Treatment is difficult because hexavalent chromium exists in the form of either chromate or dichromate depending on the ph of the wastewater. Both are negatively charged ions that cannot be readily removed unless chemjcally reduced to the trivalent state. Chromium reduction, followed by hydroxide precipitation with other metals, is the established method of treatment. Chromium reduction is normally conducted as a separate process step at low ph (less than 3) using a reduced sulfur compound, such as sodium metabisulfite or sulfur dioxide, to contribute electrons to the reaction. Treatment complexity results from (a) the need either to segregate wastes or to treat all of the wastes for chromium reduction, and (b) the need to maintain separate acidic and alkaline processes. The quantity of chemical used for ph adjustment can be significant, especially for highly buffered wastes, due to the need to first acidify, then raise the ph, ard finally neutralize the waste before discharge. Alternatively, an effective alkaline chromium reduction process can be consolidated with hydroxide precipitation, eliminating the need for waste segregation and greatly reducing the need for acid and caustic addition. This efficiency can be accomplished by alkaline reduction and precipitation using ferrous iron. The process is especially attractive for the treatment of mixed metal wastes containing low concentrations (less than 10 mg/l) of hexavalent chromium, or for neutral to alkaline wastes that are highly buffered. ALKALINE CHROMIUM REDUCTION In the past, jt has been commonly accepted that acidic ph is required for reasonably rapid chemical reduction of hexavalent chromium to the trivalent form. However, Higgins et a1.(1-3) demonstrated that ferrous reduction of hexavalent chromium is 432

2

3 The use of ferrous iron as a reducing agent improves removal efficiencies by coprecipitation effects. To improve removal further, researchers (5) have suggested using sulfide precipitation in place of hydroxide precipitation. This is based on the theoretically lower solubilities of some metal sulfides compared to those of metal hydroxides. Figure 2 shows the theore ical -6 concentrations of plating metals in equilibrium with 10 molar total sulflde and their metal sulfide precipitates. The effects of metal hydroxide complexes have been considered in contrast to previously published6(5) metal sulfide solubility curves. A concentration of 10 molar sulfide was used because it is an order of magnitude lower than that found to cause minimal odor problems (5), but still sufficient to control metal solubilities. Sulfide has the potential of chemically reducing hexavalent chromium, as well as decreasing the solubilities of other metals. The solubilities of trivalent metal sulfides, such as chromium, are greater than those of their hydroxides, so sulfide precipitation does not improve their removal. Using sulfide alone to reduce hexavalent chromium is only effective at acidic ph (2). However, operation at acidic ph is not recommended, due to the formation of hydrogen sulfide, a noxious and toxic gas. Hydrogen sulfide is the predominant sulfide species present below ph 7 (Figure 3) PH FIGURE 3. Distribution of sulfide species with ph. LABORATORY STUDIES To evaluate the combined effects of ferrous reduction and sulfide precipitation on a batch reactor, studies were performed on synthetic plating wastes, as illustrated in Figure 4, using methods previously described (1). The following are representative results obtained using mixed metal wastes, with either reagent-grade chemicals, or actual plating baths, and using hard, soft, or distilled water. Chromium concentrations varied 434

4 435

5 Sulfide alone was not effective in reducing and precipitating hexavalent chromium at neutral to alkaline ph (Figure 5). Interestingly, with the use of sulfide alone, overdosing can actually result in increased metal solubility when treating a soft water, as illustrated in Figure 6. In this case, cadmium removal was poorer with sulfide precipitation than with just hydroxide precipitation (sulfide dose = 0 meq/l) using no iron coagulant. This is believed to be due to either complexation or formation of an extremely fine cadmium sulfide precipitate that passed through the 0.45-micrometer pores of the filter used. The problem of increased solubility of metal ions with high sulfide doses was not encountered when ferrous iron was added concurrently. Ferrous reduction of hexavalent chromium with precipitation and removal of the resulting trivalent chromium has proven to be rapid and effective at neutral to alkaline ph. Stoichiometric chromium removal has been demonstrated for a wide range of conditions (1-3). In addition to being effective by itself in reducing chromium at neutral ph, ferrous iron was found to catalyze sulfide reduction of hexavalent chromium. As shown in Figures 7 (ph 7) and 8 (ph 8.5), sulfide stoichiometrically O6 -- Fa++ DOSE ImeqILl "a SYMBOL SULFIDE imw FIGURE 7. Chromium treatment by ferrous sulfate and sodium sulfide at PH 7 Fe++ DOSE (meqili 0435 FIGURE 8. Chromium treatment by ferrous sulfate and sodium sulfide at ph 8.5. improves hexavalent chromium reduction and removal when added concurrently with ferrous iron. Without ferrous iron addition, sulfide was ineffective. Under more alkaline conditions (ph 10, Figure 9). ferrous iron was found to react stoichiometrically, but sulfide addition contributed little and was ineffective by itself. 06 SYMBOL I, $ ~ g 02- b SULFIDE (meqlli 0 - J Fe++DOSE fmeqlll 1 r 1 A - $ 06 z O 4 2 t 0 z : 3 p 02 I SYMBOL EDTA (mmi FIGURE 9. Chromium treatment by ferrous sulfate Fe++ DOSE fmeqll) and sodium sulfide at ph 10. FIGURE 10. Effect of EDTA on chromium treatment by ferrous sulfate at ph 8.5. Although no improvement in chromium removal effectiveness was found at high ph with the addition of sulfide over that with ferrous iron alone, a potentially significant advantage to the concurrent use of sulfide and ferrous iron exists. Higgins and Sater (1) found that elemental sulfur is produced when sulfi2e is oxidized, compared with ferric hydroxide produced when iron is used. This translates to a sulfur sludge production of approximately 15 percent of an equivalent amount of ferric hydroxide sludge. Complexing agents, such as EDTA and cyanides, are commonly found in cleaners and plating formulations and can have an adverse effect on treatment for removal of metals. For this reason, the effects of these complexing agents on the alkaline reduction of chromium by ferrous sulfate and sodium sulfipe were investigated. Figure 10 shows that EDTA has some effect on chromium reduction with wastes produced in distilled water. Similar effects were noted for the catalytic effect of sulfide. This inhibitory effect was reduced in treating wastes prepared with a hard tap water. L Cyanide was also inhibitory to chromium reduction by ferrous sulfate (Figure 11). Again, use of hard tap water reduced the I inhibitory effects (Figure 12). In view of this inhibitory i effect and also because the resulting ferrocyanide complexes make subsequent cyanide destruction extremely difficult, it is recommended that cyanide-containing wastes be treated for cyanide destruction prior to chromium reduction by ferrous iron

6 06 p 04. I- a z w b 0' SYMBOL NaCN ImMI Fe DOSE (meqil1-0 SYMBOL NaCN (mml Fe DOSE (msq/l) FIGURE 11 EffectofcyandeoncnromLm FIGURE 12. Effect of cyan.de on cnromium iird~ment ai ph E 5 *n dist I ed watel treatment n naro tap water at PH 8 5 This pilot plant was operated intermittently for 6 months, treating a variety of synthetic plating rinsewater wastes that were prepared using either plating bath chemicals or reagentgrade chemicals, and mixed with either a hard tap water or distilled water. Typical influent and effluent compositions are shown in Table 1. TABLE 1. SUMMARY OF DATA FOR PILOT PLANT RUNS INFLUENT EFFLUENT TYPICAL RANGE TYPICAL RANGE... CHROMIUM (mg/l) <0.1 ~ a CADMIUM (mg/l) < NICKEL (mg/l) < b PH FERROUS DOSE (X of stoichiometric) SLLFIDE DOSE (X of stoichiometric) NOTES: a. The higher effluent chromium concentration occurred when the combined ferrous plus sulfide dose was only 90 percent of the stoichiometric requirement and when the waste contained 50 mg/l chromium. b. Due to cyanide interference, little if any nickel. removal occurred when treating wastes prepared with cadmium cyanide baths.,*- CHEMICAL FEED SYSTEM WASTE SYNTHESIS TANK RENCH-SCALE TREATMENT STUDIES FLOCCULATORS CLARIFIER FILTER BACKWASH SYSTEM FIGURE 13. Schematic of bench-scale pilot plant. A 5-gpm bench-scale pilot plant was built as a test bed to develop a treatment system using alkaline reduction/precipitation with ferrous sulfate and sodium sulfide addition, gravity sedimentation, and upflow filtration. A flow sheet of this treatment system is shown in Figure Figures 14 and 15 are breakthrough curves for a typical run of the pilot plant. These curves show concentrations of total metals (soluble plus precipitate) that have penetrated to different depths of the filter media. Sample Gravel+2 was taken from the filter sand 2 feet above the supporting gravel. For this run, waste was prepared by diluting chromium and cadmium plating baths with hard tap water to make chromium and cadmium concentrations of 8 and 4 mg/l, respectively. This waste was treated with ferrous sulfate and sodium sulfide at stoichiometric dose ratios of 1.0 and 0.6. Solids removal was by direct filtration. Findings from this and similar treatment runs are as follows: o Hexavalent chromium can be rapidly and effectively reduced and precipitated by a combination of ferrous sulfate and sodium sulfide at neutral and alkaline ph (7 to 10). o In-line mixing of chemicals provided sufficient reaction time for chromium reduction and metals precipitation. o The rapidity of reaction between hexavalent chromium and ferrous sulfate makes automatic control of ferrous sulfate and sulfide dosing feasible. These two could be added at a fixed ratio to each other. The rapid mix tank or in-line mixer could be monitored for hexavalent chromium and ph and feed chemical adjusted accordingly. It was 439

7 -. c found that control of ph and hexavalent chromium immediately following rapid mix allowed final effluent concentrations of chromium and other metals to be accurately predicted. o Cyanides present in plating baths (cadmium) were not in sufficient concentrations to interfere with chromium and cadmium removal, but significantly affected nickel removal. Cyanide destruction is necessary prior to treatment with ferrous sulfate. o Floc formation was so rapid that flocculation after initial mixing was not beneficial. A 20-minute flocculation period caused breakup of flocs. After chemicals were added, turbulence had to be avoided before solids removal to prevent this effect. U I IS I ' IKEY I I RUN TIME (MINUTES) FIGURE 14. Chromium breakthrough curve for benchscale pilot plant. RUN TIME (MINUTES) GRAVEL+2 GRAVEL+3 0 FIGURE 15. Cadmium breakthrough curve for bench-scale pilot plant. 440 o Upflow filtration was effective in providing solids removal with a large solide'storage capacity. Failure was dramatic, however, due to lifting of the media when pressure drop exceeded the weight of the media. o Filtration was sufficient by itself, although high concentrations of precipitated metals resulted in short run times and a high ratio of backwash water to treated water. o Addition of a clarifier extended the filter run time. Filter run times were much longer when treating distilled or soft water than when treating a hard water. The Air Force Engineering and Services Center is currently funding construction of a 10-gpm pilot plant at Tinker AFB, using alkaline ferrous and sulfide treatment of mixed metal plating rinsewater. This plant is currently in the early stages of testing. Results will be presented at a later date. APPLICATIONS Use of ferrous salts for chromium reduction and removal could be beneficial for a variety of situations. In particular, ferrous reduction appears to be most applicable for treating wastewaters containing low concentrations of chromium and other metals (e.g., mixed plating rinsewaters or cooling tower blowdown) when the discharge is also regulated by stringent effluent standards. Under these circumstances, it is often necessary to add an iron salt as a coagulant to enhance metals removal. It is difficult to precipitate and remove low concentrations of metals from solution. Addition of iron salts has been found to improve heavy metals removal through coprecipitation and adsorption. Normally, ferric salts are used for this purpose. However, ferrous salts are readily oxidized to the ferric state by hexavalent chromium or dissolved oxygen and therefore can be used effectively as coagulants. An advantage has been claimed for ferrous salts in that complete mixing is easily accomplished before oxidation of the ferrous iron to the ferric form. Ferrous salts added to accomplish coagulation of metals could be sufficient to completely reduce hexavalent chromium to its trivalent form as long as the concentration remains low (normally less than approximately 5 mg/l). In such cases, substitution of ferrous salts for ferric salts added for coagulation would provide automatic hexavalent chromium reduction as needed. This implies another advantage when chromium is present in the wastewater at concentrations requiring only intermittent removal. If a separate chromium reduction process is used, it still must be operated continuously even though it may be frequently unnecessary. Alternatively, a complicated monitoring system would be needed for automatic hexavalent chromium detection and waste diversion into a sidestream batch processing tank for chromium reduction. The continuous use of ferrous salts would eliminate both of these complicated and undesirable alternatives. If higher concentrations of hexavalent chromium are present, reduction using ferrous salts would be less advantageous. The ferrous dose required for reduction is proportional to the 441

8 concentration of hexavalent chromium in the Wastewater. If more iron is required to accomplish chromium reduction than for coagulation, use of ferrous salts rather than traditional sulfur-containing compounds would increase sludge production. Likewise, wastewaters containing high concentrations of metals are less likely to require the addition of iron salts as coagulant. With these wastewaters, the use of ferrous salts for the reduction of chromium could substantially increase sludge production and disposal costs. EXAMPLES CASE 1. To the authors' knowledge, the only full-scale operating example of alkaline chromium reduction using ferrous iron is at a Department of Defense ammunition plant located in the northeastern United States. Wastewater at this facility is generated by a metal finishing shop and a printed circuit board fabrication shop. The industrial waste treatment plant was designed to treat chromium-bearing wastes by acidifying to ph 2.5 and adding sodium metabisulfite to chemically reduce any hexavalent chromium to the trivalent form. These wastes are then combined with other waste streams for removal of metals by soluble sulfide precipitation at alkaline ph. When the chromium reduction tank sprang a leak, the Operators bypassed it and discharged directly into the main treatment system. The combination of ferrous and sulfide treatment was sufficient to enable the facility to meet its chromium effluent limitations using this alternate treatment scheme. CASE 2. Recently, an evaluation was conducted to upgrade and expand the treatment plant serving a large telecommunications research facility in the northeastern United States. The existing plant was designed to reduce hexavalent chromium at a low ph using sodium metabisulfite in a batch treatment system. Following reduction, the ph of the wastewater is raised to precipitate the mixture of metals. Ferric sulfate is added as a coagulant. Because of the results of the research, it is expected that extremely stringent chromium limitations (0.09 mg/l) may be placed on the discharge of this pretreatment facility. However, chromium is present in the discharge only intermittently and then only at relatively low concentrations. Yet when chromium is present, much of it is in hexavalent form and chemical reduction will be necessary to meet stringent standards. An evaluation of alternatives revealed this system to be a prime candidate for alkaline chromium reduction. The major advantage of using ferrous salts for chromium reduction is that the expanded treatment facilities would be simplified. It is estimated that ferrous salt concentrations added to accomplish coagulation of metals would also be sufficient to completely reduce the hexavalent chromium. Consequently, substitution of ferrous sulfate for ferric sulfate as a coagulant would provide automatic chromium reduction whenever chromium is present. A separate chromium reduction system would not be necessary and continuous chromium monitoring would not be required. 442 CASE 3. Ferrous iron addition'is not expected to be universally effective, as in this example. Jar tests were conducted on a cooling tower blowdown wastewater containing 8 mg/l of total chromium. Makeup water for the cooling tower was treated wastewater from a demonstration coal gasification process. This water contained appreciable concentrations of organic and inorganic compounds, including high concentrations of cyanide and nitrite. When iron was added to meet the stoichiometric requirements, 80 percent of the chromium remained in soluble form. Soluble chromium was reduced to 50 percent of the original concentration when 2.75 times the stoichiometric requirements were added. It was observed that as the iron dose increased, the incremental chromium removal diminished. Results also indicated that as more ferrous iron was added, more iron remained in solution as ferrous iron. It is hypothesized that cyanide or another unidentified compound present in the waste was complexing with the ferrous iron, rendering it ineffective for the chemical reduction and removal of chromium. ACKNOWLEDGEMENTS The laboratory and bench-scale testing described in this paper was supported in part by contracts and grants from the C.S. Air Force Engineering and Services Center, Tyndall AFB, FL. REFERENCES 1. Higgins, T. E. and V. E. Sater. "Combined Removal of Cr, Cd, and Ni from Wastes," Environmental Progress, 3(1):12-25 (1984). 2. Higgins, T. E. and S. G. TerMaath. "Treatment of Toxic Metal Wastewaters by Alkaline Ferrous Sulfate and Sodium Sulfide for Chromium Reduction, Precipitation and Coagulation," Proceedings of the 36th Industrial Waste Conference, Purdue University, (Ann Arbor Science Publishers, Anr Arbor, MI) : (1982). 3. Higgins, T. E. and V. E. Sater. "Treatment of Electroplating Wastewaters by Alkal.ine Ferrous Keduction of Chromiun and Sulfide Precipitation," Air Force Engineering and Services Center Report ESL-TR-83-21, 112 pp. (1983). 4. Jenne, E. A. "Controls on Mn, Fe, Co, Ni, Cu, Zn Concentrations in Soils and Water: The Significance of the Hydrous Mn and Fe Oxides," Trace Inorganics in Water, Advances in Chemistry Series No. 73, ACS (1964). 5. "Control and Treatment Technology in the Metal Finishing Industry--Sulfide Precipitation,'' U.S. EPA Technology Transfer Sumnary Report, EPA 625/ (1980). WDM09/ ,

9 New Membrane Technology in the Metal Finishing Industry Werschulz, P. P., Chanical And Related Life Whnologies, Carltech Associates, Inc., Cranford, NJ TOXIC & HAZARDOUS WASTE: Proceedings Seventeenth Mid-Atlantic Industrial Waste Conference (LaGrega, M. D., and Kugelman, I. J.) Technunic Publishing Company, Inc. (1985) This paper reviews the state of developnent of membrane technology and its application to metal finishing wastewater treatment. Metal finishing wastewaters, particularly those from electroplating rinses, can contain toxic heavy metals such as cadmium, chromium and lead. The two camon treatments used are Precipitation and ion exchange. The former, by precipitating the metal ions, produces a sludge that has to be disposed of in a Secure landfill. In addition to the sludge, there are problans with chelating agents present in the rinse water that inhibit precipitation. Flocculants are sanetimes added to aid in filtration. These significantly increase the bulk of the sludge. Ion exchange resins, while not producing a sludge, must be regenerated with strong acids or basee.?he regenerating solution then must be handled for reprocessing or disposal. New technology is emerging that makes zero sludge, zero discharge, and recycling of metal possible. First, this paper will characterize the rinse waters in the rnetal finishing industry. Particular gllphasis will be on electroplating processes. Current membrane usage will be discussed, including a brief definition and explanation of the many types of branes available for this application. The canposition and configurations of those carmercially available will be listed. Following these merging technologies, such as novel ionexchange membrane systm, inmobilized liquid manbranes and a unique reverse osmosis &rime configuration will be covered. The composition and configurations of the systems will be explained. How these membrane system may be able to overcane some of the inherent problew with the more traditional treatment processes will be discussed. KEYWORDS: Electroplating Waste, Menhranes, Reverse Oanosis, Ultrafiltration, Liquid Menbranes, Ion Exchange Manbranes 444