An Innovative Carbonate Coprecipitation Process for the Removal of Zinc and Manganese from Mining Impacted Waters

Transcription

1 ENVIRONMENTAL ENGINEERING SCIENCE Volume 24, Number 7, 2007 Mary Ann Liebert, Inc. DOI: /ees An Innovative Carbonate Coprecipitation Process for the Removal of Zinc and Manganese from Mining Impacted Waters Philip L. Sibrell, 1* Marissa A. Chambers, 2 Andria L. Deaguero, 2 Thomas R. Wildeman, 2 and David J. Reisman 3 1 USGS Leetown Science Center Kearneysville, WV Department of Chemistry and Geochemistry Colorado School of Mines Golden, CO ORD Engineering Technical Support Center MLK-489 Environmental Protection Agency Cincinnati, OH ABSTRACT Although mine drainage is usually thought of as acidic, there are many cases where the water is of neutral ph, but still contains metal species that can be harmful to human or aquatic animal health, such as manganese (Mn) and zinc (Zn). Typical treatment of mine drainage waters involves ph adjustment, but this often results in excessive sludge formation and removal of nontoxic species such as magnesium and calcium. Theoretical consideration of the stability of metal carbonate species suggests that the target metals could be removed from solution by coprecipitation with calcium carbonate. The U.S. Geological Survey has developed a limestone-based process for remediation of acid mine drainage that increases calcium carbonate saturation. This treatment could then be coupled with carbonate coprecipitation as an innovative method for removal of toxic metals from circumneutral mine drainage waters. The new process was termed the carbonate coprecipitation (CCP) process. The CCP process was tested at the laboratory scale using a synthetic mine water containing 50 mg/l each of Mn and Zn. Best results showed over 95% removal of both Mn and Zn in less than 2 h of contact in a limestone channel. The process was then tested on a sample of water from the Palmerton zinc superfund site, near Palmerton, Pennsylvania, containing over 300 mg/l Zn and 60 mg/l Mn. Treatment of this water resulted in removal of over 95% of the Zn and 40% of the Mn in the limestone channel configuration. Because of the potential economic advantages of the CCP process, further research is recommended for refinement of the process for the Palmerton water and for application to other mining impacted waters as well. Key words: acid mine drainage; limestone coprecipitation; carbon dioxide; alkalinity; metal removal; manganese; zinc *Corresponding author: USGS Leetown Science Center, Leetown Road, Kearneysville, WV Phone: (304) ; Fax: (304) ; psibrell@usgs.gov 881

2 882 INTRODUCTION ACID MINE DRAINAGE (AMD) is an unintended consequence of decades to centuries of mining that occurred before the advent of environmental regulation. Despite the implication of the term acid mine drainage, many mining-related sites have discharges that are not acidic at all, but are circumneutral in ph (Rose and Cravotta, 1998). However, these neutral waters often contain elevated levels of metals harmful to aquatic life, including zinc (Zn), cadmium (Cd), and manganese (Mn) (Earl and Callaghan, 1998). The metals can be removed by addition of an alkaline agent to bring the ph to 10, followed by settling or filtration to remove the precipitates, and readjustment of the ph for discharge into the environment. This process has the drawback of generating voluminous sludges that require expensive handling and disposal. Reagent costs are high because lime or sodium hydroxide is typically required to reach ph 10. In addition, the ph readjustment requires further processing of the solution. A method that would remove these harmful metals without requiring lime or sodium hydroxide addition, generating large quantities of sludge and requiring ph readjustment would be a vast improvement in AMD treatment technology. A process consisting of coprecipitation of the metals Zn, Cd, and Mn in a calcium carbonate (calcite) matrix has been observed by one of the authors under natural conditions in the field. Preliminary laboratory tests have reproduced this phenomenon. The purpose of this work is to conduct a laboratory testing program to find the optimum conditions for the removal of the harmful metals through this novel carbonate coprecipitation (CCP) process. The coprecipitation of Mn and Zn with calcite has been investigated by several research groups interested in the geochemical behavior of these elements. Wildeman (1969) used electron paramagnetic resonance to determine the chemical environment of the Mn in carbonate mineral samples. His results showed that the Mn was incorporated into lattice sites of the carbonate structure. Pingatore et al. (1988) investigated coprecipitation of Mn with calcite and found that the distribution coefficient of Mn precipitated in calcite is dependent on the calcite precipitation rate, in accordance with findings by earlier investigators (Lorens, 1981), and that the distribution coefficient decreased as the rate of precipitation increased. Based on electron spin resonance analysis of the coprecipitated calcite, they also concluded that the Mn was incorporated into the calcite as a solid solution rather than as a separate phase such as MnCO 3. However, other investigators have found other mineral phases present in coprecipitated samples. Franklin and Morse (1983) studied Mn uptake by calcite and concluded that Mn is first SIBRELL ET AL. absorbed on the calcite surface, then MnCO 3 nucleates, and grows on the calcite surface. This reaction was slowed significantly by the presence of Mg at levels approaching that of sea water. Mucci (1988) investigated Mn uptake by calcite in seawater, and identified the precipitated compound as pseudo-kutnahorite or MnMg(CO 3 ) 2. Jun et al. (2005) studied nucleation and growth of manganese oxides on the surfaces of carbonate minerals, and found that epitaxial growth of manganese oxide occurred on MnCO 3 and MgCO 3, but not on CaCO 3 surfaces. Zachara et al. (1991) investigated the sorption of several transition metals on calcite surfaces, and found that Zn and Co were readily and reversibly adsorbed, while Mn and Cd were readily, but irreversibly, adsorbed. This was related to the ionic radii and to the degree of hydration of the adsorbed complex. In an investigation on the precipitation of Zn with calcite Zachara et al. (1989) found that the Zn was not incorporated as a solid solution, but was precipitated on the calcite surface as the mineral hydrozincite, with the composition Zn 5 (OH) 6 (CO 3 ) 2. These studies establish that removal of Zn and Mn from solution via coprecipitation does indeed happen, but the nature of the precipitate varies and may depend on experimental conditions. Application of coprecipitation processes to treatment of contaminated waters has not been as extensively investigated. Aziz and Smith (1996) investigated the use of a limestone rock filter for removal of low concentrations of Mn from drinking water. Mn removal depended on the type of seed material used, with limestone being superior to gravel or crushed brick. However, no analyses were performed to determine the composition of the precipitate. Current technology for Mn removal from AMD is based on passive systems composed of horizontal trenches of limestone through which the AMD is passed. The precipitate recovered from these beds appears to be the manganese oxide todorokite (Rose et al., 2003), rather than a carbonate. However, these waters rarely contain elevated levels of Ca and bicarbonate alkalinity, so that carbonate minerals would not be expected to form. For waters with elevated Ca and bicarbonate concentrations, carbonate coprecipitation could be a viable alternative to oxide precipitation depending on the rate and extent of the precipitation reaction. The carbonate coprecipitation phenomenon was observed to occur naturally by one of the authors in water draining from a mine site near the town of Rico, in southwestern Colorado. Three adits to the west of the city had phs around 7, contained significant concentrations of Zn, and had high levels of bicarbonate alkalinity. At one of the adits, a trench that was built to direct the water to a receiving pond was lined with concretions that obviously precipitated from the adit water. The concretions dis-

3 REMOVAL OF ZINC AND MANGANESE FROM MINING WATERS 883 solved in nitric acid with considerable effervescence, indicating that they were composed at least partly of carbonate minerals. The water chemistry at the end of the trench showed that calcium was reduced in concentration, the ph increased, and metal contaminants were reduced to concentrations below aquatic toxicity limits. This observation shows that in a natural precipitation of calcite from neutral waters, removal of metal contaminants was extensive enough to render the water safe for aquatic organisms. These results are supported by an examination of the solubility products (K sp ) of the carbonate minerals involved in the process. The pk sp values for Ca, Mn, Zn, and Cd carbonates are 8.3, 10.65, 9.92, and 11.21, respectively (Lide, 1993). Thus, under equilibrium conditions, each of the Mn, Zn, and Cd carbonate minerals will precipitate preferentially to calcite. For the CCP process to work effectively, a significant amount of calcium carbonate must be dissolved in the water. Although this does occur in nature at least some of the time, in other cases, calcium carbonate or alkalinity will need to be added to the water. This can be readily accomplished using the pulsed limestone bed (PLB) process developed by researchers at the U.S. Geological Survey s Leetown Science Center (LSC) in Kearneysville, West Virginia, for treatment of acidic mine drainage (Watten and Schwartz, 1996; Sibrell et al., 2000; Sibrell et al., 2003). In the PLB process, limestone (calcium carbonate) is dissolved under pressure of carbon dioxide, and this gives the water an excess of bicarbonate alkalinity. Then calcite can be precipitated by air stripping the water to remove the carbon dioxide. Because the product from the treatment process is calcite with abundances of metals that are about the same as in natural limestone, there are only minor disposal issues. Also, since Figure 1. Effect of CO 2 and temperature on equilibrium alkalinity for limestone in water. Figure 2. Effect of CO 2 on limestone dissolution kinetics in pure water at 25 C. limestone is the least expensive reagent available for the neutralization of acidity (Hedin et al., 1994), the process is less expensive than using sodium hydroxide or lime. The PLB system depends on enhancement of the rate of limestone dissolution through addition of carbon dioxide (CO 2 ). As a carbonate mineral, limestone is strongly affected by the partial pressure of CO 2. Figure 1 illustrates the effect of increasing the CO 2 pressure on limestone solubility, in terms of alkalinity in mg/l as CaCO 3. Solubilities were calculated at three different temperatures using the PHREEQC geochemical modeling program (Parkhurst, 1995). The calculations are confirmed by experimental data from Lovell (1973). Due to the mathematics of the equilibrium equations, the alkalinity increases as the cube root of CO 2 pressure (Stumm and Morgan, 1996). Thus, a shift from atmospheric CO 2 at atm to a modest pressure of 30 kpa ( atm) results in a 10-fold increase in equilibrium solubility, from 50 to 500 mg/l as CaCO 3 at 25 C. As the temperature decreases, CO 2 solubility increases, thus increasing alkalinity as well, under equilibrium conditions. The kinetics of limestone dissolution are also affected by CO 2. Limestone can be dissolved according to the following mechanisms: CaCO 3 H Ca 2 HCO 3 (1) CaCO 3 H 2 O CO 2 Ca 2 2HCO 3 (2) CaCO 3 H 2 O Ca 2 HCO 3 OH (3) These mechanisms are termed attack by acid, CO 2, and water, respectively (Plummer et al., 1978). Under certain conditions of ph and CO 2 pressure, one or another of these mechanisms may be dominant. The total dissolution rate is the sum of these three mechanisms. Figure 2 shows the contributions of each of the mechanisms to the ENVIRON ENG SCI, VOL. 24, NO. 7, 2007

4 884 dissolution of calcite in initially pure water as a function of CO 2 pressure. The rates were calculated based on rate constants determined experimentally for water at 25 C. The rate of reaction (3) is not affected by CO 2, so it appears as a horizontal line on the plot, fixed only by temperature. The rate of CO 2 attack, reaction (2), increases with CO 2 pressure, and exceeds the water rate at a pressure of about 0.1 atm. The acid attack rate, reaction (1), is also affected by CO 2 addition, because CO 2 is a weak acid, through a shift of the ph to lower levels as CO 2 pressure increases. Under the conditions given, however, this mechanism does not contribute significantly to the overall dissolution rate. Note that if excess mineral acidity is present in the water, as would be typical of many AMD sites, the acid attack rate would be accelerated. The net effect of CO 2 addition is an increase in dissolution rate above a CO 2 pressure of 0.01 atm, with a doubling of the rate at 0.1 atm, and an enhancement of up to 10 times the water rate at 1 atm pressure. In summary, increasing the CO 2 pressure increases both the rate and extent of limestone dissolution, rendering the PLB system a practical means for calcium carbonate saturation. Removal of CO 2 by air stripping reverses reaction (2), resulting in the precipitation of calcium carbonate, and coprecipitation of other metal carbonate species. Thus, manipulation of the CO 2 pressure is the key to the coprecipitation process, through control of the solubility of the metal carbonate species. METHODS PLB system description A schematic of the PLB treatment apparatus is shown in Fig. 3. A treatment apparatus sized to treat up to 1 L/min of flow consists of four 2.5-cm (1-in) diameter plastic columns containing limestone, and one 5-cm (2- in) diameter column functioning as a packed tower for CO 2 absorption into the water (the carbonator). Incoming water is routed to one set of two columns (columns 3 and 4, as depicted in Fig. 3) containing limestone. The limestone particle size is roughly 0.1 to 1.0 mm. The flow fluidizes a single limestone bed for a period of 1 min, and then the flow is diverted to the other column for 1 min, while the bed in the first column settles. Water is discharged to the drain continuously. This pulsed-bed operation allows higher flow rates to be passed through the limestone bed, thus providing for better mixing and scouring of the particle surfaces. The flow path is controlled by electrically actuated solenoid valves operating on a timer-controlled program. Meanwhile, the water in the other set of two columns (columns 1 and 2 in Fig. 3) is recirculated through the Figure 3. SIBRELL ET AL. Schematic of pulsed limestone bed system. carbonator, where CO 2 can be added to the water. This is termed the recycle mode, and the water is diverted back and forth between two limestone columns as before on 1-min intervals. The water in the recycle section can be considered as a batch treatment under elevated CO 2 pressure that lasts for a total of 4 min. At this point, another set of solenoid valves is actuated, and the columns that had been receiving incoming water are switched to recycle mode, and the columns that had been on recycle are switched to charge and discharge water. The water that is discharged from the top of the reactor is predominantly treated water, while fresh, untreated water is charged into the bottom of the reactor for another cycle of treatment. Thus, at any one time, one out of the four columns is receiving fresh influent and discharging treated water from the previous recycle phase, and one of the columns is receiving water recycled through the carbonator. Carbon dioxide was directed from a high-pressure gas cylinder through a mass flow meter and into the carbonator column of the treatment system. An Aalborg mass flow meter gave CO 2 flow in standard liters per minute (SLPM), that is, the gas volume was corrected to standard conditions of 294 K (21.1 C) and 1 atmosphere pressure. The limestone in the reactors was consumed by neutralization of acid and by generation of alkalinity. The initial charge was about 210 g of limestone to each reactor, and additions were made to each column before each test to maintain a settled limestone bed depth of about 30 cm. The source of the limestone used in the test system was the Bellefonte Limestone Company, near Bellefonte, Pennsylvania. (The limestone composition is given in Table 2, as scale A.) The overall system volume was about 4.7 L, taking into account the volume of all four columns (1.0 L each), the carbonator, the connecting tubing, and accounting for the volume take up by the limestone sand.

5 REMOVAL OF ZINC AND MANGANESE FROM MINING WATERS 885 Testing and analysis The first objective of the program was to run a complete set of process tests to evaluate the effects of CO 2 pressure, carbonator water level, and treatment flow rate on PLB performance for preparation for the coprecipitation process. These tests were run with spring water available at the LSC. Performance was measured by effluent ph and alkalinity. Carbon dioxide pressures were 41, 69, 83, and 103 kpa, water level varied from 18 to 71 cm, and treatment flow rates tested were 0.3, 0.5, and 0.7 L/min. Water temperature was 23 2 C. Water quality analyses were performed in accordance with standard protocols (APHA, 1995). Analyses performed on-site included ph, temperature, alkalinity (Method 2320B), and acidity (Method 2310B, with hot peroxide treatment). Both alkalinity and acidity results were reported in units of mg/l as CaCO 3. A duplicate of each effluent sample was vigorously air stripped for 7 min using a diaphragm pump to remove CO 2, and the ph again measured. The CO 2 concentrations in the influent and effluent samples were determined, if required, by titration to ph 8.3 (Method 4500-CO 2 -C). Alkalinity was used as the measure of calcium carbonate dissolution in the PLB performance tests. The area where the LSC is located is a wellknown karst region, with extensive carbonate mineral formations. Spring water at the LSC contains about 280 mg/l alkalinity and was used as a starting point for the carbonate dissolution part of the study. The effect of feed water quality was also tested, by processing other feed waters, including deionized water, and spring water with acid and metal additions. These tests were conducted under standard conditions of 69 kpa CO 2 pressure, 23 cm carbonator water level, and flow rates of 0.5 L/min in both the influent and recycle lines in the PLB. As shown by reaction (2), air stripping of the PLB effluent removes CO 2, thus increasing the ph and the saturation index of the calcium carbonate. For example, a typical PLB effluent might have an alkalinity of 400 mg/l at ph 7.0, but water in equilibrium with calcite and CO 2 in the atmosphere (at 25 C) has a ph of 8.3 and an alkalinity of only 50 mg/l (Stumm and Morgan, 1996). Therefore, the CCP process is implemented by air stripping to remove CO 2 in the presence of calcite seed material. Initial process testing for removal of Mn and Zn was conducted using a synthetic miningimpacted water consisting of 50 mg/l each of Mn and Zn added as metal sulfate salts to LSC spring water (Table 1). The PLB operating conditions for the coprecipitation tests were chosen to give high levels of alkalinity and calcium to the solution, by processing at an elevated P CO2 of 103 kpa and a flow rate of 0.3 L/min for increased retention time. Table 1. samples. Composition of synthetic and Palmerton raw water Water sample Analyte Synthetic Palmerton ph Alk Al Ca Cd B.D.L Cu B.D.L Fe K Mg Mn Na Zn Cl NO 3 -N SO Sample ph is reported in standard units, alkalinity (Alk) is in mg/l as CaCO 3, and all other concentrations are in mg/l. A report of B.D.L. indicates concentration below detection limit. The rate and extent of precipitation reactions are often enhanced through the addition of seed crystals to form a surface upon which the precipitation reaction can take place. Three different seed materials were tested: Bellefonte limestone (Scale A), calcium carbonate water scale scraped from the side of a holding tank (Scale B), and degasser scale from a vacuum degasser at the LSC site (Scale C). Each of these seed materials had different morphologies. The limestone was a dense crystalline phase; the tank scale was thin, light, and flaky, while the degasser scale was denser and more nodular. Impurity levels of the scales were determined by X-ray fluorescence (Table 2). Based on the CaO content coupled with the loss on ignition (LOI), the results for all three scales were consistent with calcium carbonate, with low levels of other rock forming elements. Samples were also analyzed by X-ray diffraction to identify the mineral phases present. Both the limestone and tank scales samples were identified as major calcite with a trace of silicate impurity. The degasser scale showed major calcite and aragonite, an alternative crystal structure of calcium carbonate. The metal ions in the calcite structure are coordinated with six oxygen atoms from the carbonate groups, whereas in aragonite, the metal is coordinated with nine oxygen atoms (Hurlbut and Klein, 1977). Since both rhodochrosite (MnCO 3 ) and smithsonite (ZnCO 3 ) are structurally similar to calcite, scales A and B may be ex- ENVIRON ENG SCI, VOL. 24, NO. 7, 2007

6 886 Table 2. Whole rock analysis of calcium carbonate scale samples by X-ray fluorescence. Scale sample Bellefonte Tank Degasser limestone, Scale, Scale, Analyte Scale A Scale B Scale C SiO Al 2 O Fe 2 O CaO MgO MnO LOI Total S Rock-forming elements are reported as % of the oxide. LOI is loss on ignition, and represents bound water, carbonates, and sulfides. The sum of the oxides and LOI should give 100%. Sulfur analyses were performed separately. SIBRELL ET AL. pected to be more suitable than scale C, with its aragonite content. Three different configurations were tested for the air stripping and contact of the water with the seed material. These were beaker, tray, and channel tests. Beaker tests were conducted by adding 1 L of PLB effluent to a plastic 2-L beaker, which was stirred magnetically and sparged with air at a rate of 2 L/min using a small diaphragm compressor. Each of the seed materials described earlier was tested at levels from no seed up to 5 g/l water. Because the seed was expected to be more effective if suspended, the seed additions for the beaker tests were ground to pass 100 mesh screen size. The progress of the reaction was monitored continuously through ph, and samples were withdrawn periodically for analysis of alkalinity by titration (APHA Method 2320A) and for Mn by colorimetry (Hach). Tray tests were conducted by adding 1 L of treated air-stripped effluent to a large stainless steel tray ( cm) resting upon an orbital shaker. Test variables in this case were the amount of seed added and the agitation rate as set by the orbital shaker. Seed crystals were not suspended during tray tests, allowing for a much larger addition of seed, of 300 to 600 g. Also, because the seed was not suspended, a coarser mix was used, with a maximum particle size of 1 mm. Not enough of the tank scale (Scale B) was available for the tray tests, so only the other two seed types were tested. Agitation rates tested were 40 and 120 revolutions per minute (rpm). Channel tests were conducted by flowing air stripped PLB effluent down a 2.5-m long rain gutter in which was spread 1.5 kg of the coarse limestone sand (Scale A). The water was collected in a bucket at the end of the channel and recirculated to the channel head using a peristaltic pump. The flow rate through the channel was set at 100 ml/min. Samples of untreated water, PLB effluent, and CCP process tests were analyzed for metals content of the water by inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Department of Chemistry and Geochemistry at the Colorado School of Mines. Metals content was determined after filtration with m membrane filters. Filtration and sample preservation (addition of nitric acid to ph 2) were performed in the field as soon as possible after sampling. For comparison to coprecipitation results, baseline ph adjustment tests were run on the feed water samples using sodium hydroxide and lime. Reagent consumption and the resulting sludge volume were measured for each of the titration scenarios. Sludge settling rate and volume were measured by adding the stirred suspension to an Imhoff Cone, then recording the height of the liquid/slurry interface after specific time intervals, usually 1 and 24 h. Coprecipitation tests were also conducted on a sample of water from the Palmerton zinc site near Palmerton, Pennsylvania. A zinc smelter has operated in this location since the early 1900s, with a resulting pile of smelter slag and residue nearly 1 mile long. Water infiltrating the pile has contaminated ground water in the area, and has Table 3. Effect of pulsed limestone bed operating conditions on effluent water quality. Carb. Influent Recycle P(CO 2 ) level flow flow Avg. Avg. Alk (kpa) (cm) (L/min) (L/min) n ph (mg/l) Spring water temperature 23 2 C. (Carb. level water level in carbonator, n number of tests, Avg. ph and Alk are the average ph and alkalinity for each group of tests.)

7 REMOVAL OF ZINC AND MANGANESE FROM MINING WATERS 887 prompted the listing of the site by the U.S. Environmental Protection Agency (U.S. EPA) on the National Priority List for remediation. A sample was obtained at the Palmerton site in August of 2005 with the cooperation of site contractors and the U.S. EPA. The analysis of the raw Palmerton water was given earlier in Table 1. These results show that the Palmerton water has much higher levels of Zn and Mn, as well as elevated levels of background salts, such as sodium, potassium, and magnesium sulfates. Sulfate was by far the most prevalent of the anionic species. PLB and coprecipitation test procedures and analysis were as described above for the synthetic mine water. RESULTS AND DISCUSSION PLB operating conditions Initial investigations focused on production of saturated solutions of calcium carbonate via the pulsed limestone bed process. As shown in Table 3, the alkalinity of treated spring water increased as the applied carbon dioxide pressure increased, as the carbonator water level decreased, and as the treatment flow rate decreased. Statistical analysis of these results was used to construct a model of the experimental alkalinity based on these parameters, with the result: Alk * P CO * Level 215 * Flow, (4) N 27, R , F 35.87, p 0.05, (4) where Alk is the alkalinity of the PLB effluent water in mg/l as CaCO 3, P CO2 is the applied pressure of CO 2 in kpa, Level is the carbonator water level in cm, and Flow is the treatment flow rate, in L/min. The fit of the model is good, as evidenced by the correlation coefficient, F- ratio and P value. These results can be explained through the interactions of the water with the limestone and CO 2 in the PLB reactors. As mentioned earlier, equilibrium alkalinity increases with the cube root of CO 2 pressure. This relationship has been previously modeled for the PLB as a square root dependency for ease in solving (Watten et al., 2004), and that form has been maintained in this study. The water level in the carbonator had an inverse relationship with effluent alkalinity. The carbonator is a packed column reactor that serves as the device for introducing CO 2 into the water during the treatment phase of the PLB operation. Lower water levels expose more of the packing, and therefore more interfacial area for the water that is trickling down through the packing in the CO 2 -rich atmosphere inside the carbonator. The water level in the carbonator can be a limiting factor at elevated CO 2 pressure, if enough area is not available for the CO 2 Table 4. Effect of water source on pulsed limestone bed performance. Alk Avg. Alk Type of water (effluent) Trial ph (mg/l) (mg/l) Spring Deionized Spring 0.88 g/l sulfuric acid Spring 1.5 g/l sulfuric acid Spring 50 mg/l each Mn and Zn Spring 50 mg/l Mn Spring 50 mg/l Zn All tests were performed with P co2 69 kpa, carbonator water level of 23 cm, and water flow rate of 0.5 L/min. ENVIRON ENG SCI, VOL. 24, NO. 7, 2007

8 888 to be dissolved into the water. Test results (Table 3) showed that water level above 23 cm resulted in decreased alkalinity because of the decreased area for CO 2 solubilization to occur. Alkalinity is also affected by flow rate through the residence time that the water spends in contact with limestone in the reactor. Lower flow rates allow for higher residence times, increasing the amount of time available for calcite dissolution to occur. Given that the total volume of the treatment system was 4.7 L, the average retention times were 15.7, 9.4, and 6.7 min at flow rates of 0.3, 0.5, and 0.7 L/min, respectively. Previous results (Sibrell et al., 2005) have shown that there is a minimum flow rate under which insufficient fluidization of the limestone bed takes place, leading to poor mass transfer and decreased alkalinity. This minimum flow rate appears to be below the 0.3 L/min flow, since alkalinity increased consistently as flow decreased. The effect of the feed water composition to the PLB was also investigated. As shown previously, when spring water was processed in the PLB, the resulting alkalinity was 450 mg/l, but the spring water started with an alkalinity of 280 mg/l, for a net increase of 170 mg/l. Deionized (DI) water processed under the same conditions resulted in an alkalinity of 340 mg/l, a lower absolute alkalinity, but a greater net change (Table 4). Under the operating conditions of the PLB, the CO 2 pressure SIBRELL ET AL. sets the maximum alkalinity that can be achieved, in accordance with the equilibrium alkalinity. However, because of the limited time available in the system, equilibrium is not always reached, and net reaction rates are greatest when furthest from equilibrium. Therefore, the DI water, starting from an alkalinity of 0, had a greater net change, while the spring water, starting from 280 mg/l, approached equilibrium more closely, which at 69 kpa CO 2, would be about 1,000 mg/l at 16 C (Lovell, 1973). In the next test series, sulfuric acid was added to spring water with the intent of generating CO 2 internally by reaction with limestone. Also, at low ph, the acid dissolution mechanism is known to be the most rapid of the three mechanisms for limestone dissolution (Plummer et al., 1978). Concentrations of Ca 2 and SO 4 2 increased considerably in the treated water, although alkalinity was not generally affected. This may be due to the fact that the CO 2 was supplied by a pressure regulator on demand, and therefore, even though less CO 2 was required from the external source, the CO 2 pressure and thus the driving force for the reaction was the same. The effect of added metal ions was tested next. Because metal-bearing waters would need to be pretreated by the PLB process, any effect due to the presence of the metals should be recognized. Results in Table 4 show Figure 4. Precipitation kinetics for synthetic solution beaker tests.

9 REMOVAL OF ZINC AND MANGANESE FROM MINING WATERS 889 that the effluent alkalinity was significantly reduced by the addition of 50 mg/l each of Mn and Zn. The metals were also added separately to spring water, and Mn was shown to have a slightly higher inhibitory effect. It is well known that metal ions adsorb on to calcite from solution (Zachara et al., 1991), and by adsorption onto the calcite substrate, the metals may interfere with the dissolution process at the liquid/solid interface. On the other hand, this may indicate that these metals, and Mn in particular, are strongly attracted to calcite surfaces where coprecipitation could take place if the solids were oversaturated. Coprecipitation synthetic solution Testing of the CCP process began with the synthetic solution described earlier. Baseline ph adjustment titrations were performed to find the amount of base required and the resulting sludge volume for treatment of the solution. The synthetic solution required 0.14 g/l of NaOH or 0.11 g/l of lime for adjustment and stabilization at ph 10. Removal of Zn and Mn was essentially complete, at 99% or higher. Sludge volume was low at 2 3 ml/l or %, because of the purity of the synthetic solution. The first air strip configuration tested was the beaker setup as described in the Experimental Methods section. Figure 4 shows the ph, alkalinity, and concentrations of Ca, Zn, and Mn as a function of time during stripping tests with and without seed addition. The ph increased rapidly in the first hour of the test as air stripping removed CO 2 from solution. The Zn was essentially completely removed over the same time period, indicating that Zn removal may be more due to hydrolysis and formation of a hydrated oxide rather than carbonate coprecipitation. Manganese was more difficult to remove and took a greater period of time. Without addition of seed crystal, the Mn removal rate was slow, and after 6 h of air stripping, only about one-half of the Mn had been removed. The Ca concentration over the same time period behaved erratically but showed little net change over the course of the test, indicating that not much calcite had precipitated. Possibly the Mn was not being removed by calcite coprecipitation at all, but through another mechanism. It is well known that Mn(II) will slowly oxidize Figure 5. Precipitation kinetics for synthetic solution tray tests. ENVIRON ENG SCI, VOL. 24, NO. 7, 2007

10 890 to form insoluble higher oxidation state oxides up to MnO 2 (Stumm and Morgan, 1996), and this could be the mechanism for Mn removal without seed addition. When calcite seed materials were added, the Mn removal rate was more rapid, with 50% Mn removal after 2 h and as much as 90% after 6 h. This effect could be due to an enhancement in the Mn oxidation rate at calcite surfaces or via a coprecipitation process. In these tests, Ca decreased more consistently, as did alkalinity, suggesting that more calcite was being precipitated and that coprecipitation was the likely removal mechanism. Scale B gave better Mn removal rates than Scale A, but differences in seed type were not consistent, since better results were sometimes noted with 1 g/l seed than with 5 g/l (data not shown). Air flow rate during the beaker strip tests was set at 2 L/min, but appeared to vary erratically, especially over the long course of the tests, and may have affected the results. The second configuration tested was the tray test, where the air stripped PLB effluent was added to a shallow metal pan with a large air water surface, and the whole assembly rotated using an orbital shaker. As before, Zn removal was rapid and was typically complete by the end of the air-stripping step or shortly thereafter. Figure 5 shows the ph, alkalinity, and concentrations of Ca, Zn, and Mn vs. time for the tray tests. It is apparent that the Mn removal was much more rapid than for the beaker tests, even those with added seed. The seed addition was much greater than that used in the beaker tests, at 300 and 600 g/l. The higher limestone seed (Scale A) addition appeared to give a small but noticeable increase in Mn removal at 40 rpm. The Ca 2 decreased more rapidly than in the beaker tests, again suggesting precipitation of calcite. The degasser seed material (Scale C) appears to be less effective than limestone. This material was nodular and may have a lower surface area than the limestone sand. The degasser seed also contained more impurities (Table 2), resulting in increased Ca 2 and SO 4 2 concentrations in solution over time. The increased agitation rate appeared to confer a small advantage in Mn removal rate, perhaps due to greater oxygen uptake at the air/water interface. The last CCP configuration tested was the channel setup. As with the tray test, the PLB effluent was air stripped before being introduced to the channel, and as before, the Zn was essentially removed by the end of the air strip step. The channel was pretreated with a 1-L sample of airstripped effluent before the test solution was contacted with the channel. Figure 6 shows ph, alkalinity and concentrations of Zn, Mn, and Ca over the course of the test. The removal rate for Mn was the most rapid of any of the airstrip configurations. After only 15 min, Mn removal was 70%, which increased to 90% after 1 h. Even after only 3 Precipitation kinetics for synthetic solution chan- Figure 6. nel tests. SIBRELL ET AL. min of contact in the limestone channel, the channel effluent had dropped to about 16 mg/l Mn, for a removal of about 70%. The Ca 2 concentration in the water dropped significantly as well, although there was an increase in the last time interval, again suggesting calcite precipitation. These results show the limestone channel to be the most effective means of Mn removal and treatment of contaminated water, perhaps because the channel contained the highest amount of limestone seed. In summary, test results indicated that the removal of Mn is strongly influenced by the quantity of seed material present. As seed quantity increases, the proportion of Mn in the precipitate decreases, which in turn, affects the thermodynamics of the precipitation reaction. This can be illustrated by examination of the standard expression for the solubility of a compound, as shown in Equation (5): MnCO 3 Mn 2 CO 2 a 3 ; K sp Mn 2 * a 2 CO3 (5) amnco3 Assuming that in dilute solution the activity of the dissolved species is approximately equal to the concentration, and solving for the equilibrium concentration of Mn 2 : [Mn 2 K sp * a ] MnCO3 (6) [CO3 2 ]

11 REMOVAL OF ZINC AND MANGANESE FROM MINING WATERS 891 When the precipitate is pure MnCO 3, the activity of the precipitate is unity, and the equilibrium concentration of Mn 2 is fixed by the ratio of the solubility product and the carbonate concentration. However, when the Mn is incorporated as an impurity in a matrix of calcite, its activity, as expressed by the product of the mol fraction X and the solid solution activity coefficient f, is less than unity, thus decreasing the solubility of the Mn [Equation (7)]. K [Mn 2 sp * X MnCO3 * f ] MnCO3 (7) [CO3 2 ] Therefore, at a fixed carbonate concentration, the lower the mol fraction of MnCO 3 in the precipitated phase, the lower the equilibrium concentration of Mn 2 must be. Because the activity of Mn in the calcite matrix decreases as the ratio of Mn/Ca decreases, the driving force for the reaction is proportional to the amount of seed. For a more detailed discussion of the thermodynamics of precipitation processes, the reader is referred to Stumm and Morgan (1996). Kinetic factors may also play a role in the removal of Mn. The rate of the Mn 2 precipitation reaction would be expected to be related to exposed surface area, which would be increased as seed addition was increased. Therefore, both thermodynamic and kinetic factors would predict an increase in Mn removal as the seed addition was increased, in accordance with the observed results. CCP process Palmerton water Adjustment of the solution ph with sodium hydroxide or lime was used to establish baseline metal removal, reagent consumption, and sludge volume for the Palmerton water. Treatment to ph 10 resulted in complete Mn and Zn removal, but consumed 0.66 g/l NaOH or 0.71 g/l lime. The sludge produced was voluminous and slow settling, with volumes of about 130 ml/l of the initial volume treated even after 24 h settling time. This is in contrast to the 2 3 ml/l observed for the much cleaner synthetic solution. Clearly, treatment of the Palmerton water by ph adjustment would incur significant sludge handling and disposal costs. Analysis of the filtered treated water showed that the Mg concentration had decreased, from about 350 mg/l in the feed solution, to 230 mg/l for the lime-treated water, and 270 mg/l for the sodium hydroxide treatment. These results demonstrate that the nonselective ph adjustment method has resulted in unnecessary reagent consumption and sludge generation because of partial hydrolysis and precipitation of Mg species at ph 10. As the tray and channel configurations showed the greatest promise during testing of the synthetic mine wa- ter solution, those two methods were further tested in the treatment of the Palmerton water. The PLB operating conditions were as before, with a flow rate of 0.25 L/min, and a P CO2 of 103 kpa. The water was then treated in the tray and channel coprecipitation setups. Complete results for the Palmerton tests are given in Table 5 and ph, alkalinity, and metal concentration vs. time are shown in Fig. 7 for selected channel tests. Initially, the water was treated as is after PLB processing (shown in Fig. 7 with a solid line). Whereas essentially all of the Zn had been removed from the synthetic solution by the conclusion of the air stripping step, about 50% of the Zn still remained in the Palmerton water. Treatment in the trays and channel further decreased Zn concentration to give 70 to 80% removal, but made only a small impact on the Mn concentration. Key factors in the different behavior observed between the Palmerton and the synthetic water are the ph and alkalinity during the coprecipitation process. Note the lower ph and alkalinities shown in Fig. 7 than for equivalent tests in Figs Due to the greater metal content of the Palmerton water, the acidity released by the hydrolysis of the Zn must have neutralized a significant portion of the excess alkalinity, thus causing a decrease in ph and alkalinity, and a decrease in the driving force for calcite precipitation and Mn removal. The PLB system added about 300 mg/l alkalinity, but the hot acidity of the Palmerton water was over 500 mg/l, indicating that insufficient alkalinity was available to neutralize the acidity generated. Therefore, in order to increase metal removal, it was necessary to add additional alkalinity to the Palmerton water. Several methods of accomplishing this were tested. In the first approach, the Palmerton water was simply diluted 1:1 with DI water to decrease the acidity and enable the PLB to supply an excess of alkalinity. Following PLB treatment, the diluted water had an alkalinity of 350 mg/l, which was in excess of its potential acidity from the Zn and Mn. Both the tray and channel contact configurations were tested for metal removal following PLB treatment. Results for the channel test are shown in Fig. 7 (dotted line). Zinc removal increased to 95% or better, but Mn removal was still below 25%. The Ca 2 concentration actually increased during channel treatment, suggesting that no calcite crystallization was taking place. In the two-pass process test, the Palmerton water was run through the PLB, processed through the limestone channel, then sent back through the PLB a second time to pick up additional alkalinity before another pass through the limestone channel. This resulted in an improvement in metal removal, to 95% for the Zn and about 30% for the Mn. Finally, in the recycle test, 15 L of Palmerton water was continuously recycled through the PLB (P CO2 103 kpa) to build up the alkalinity so ENVIRON ENG SCI, VOL. 24, NO. 7, 2007

12 892 SIBRELL ET AL. Table 5. Removal of Zn and Mn from Palmerton water after pulsed limestone bed pretreatment. % Removal Time Sample Config. (min) ph Alk Ca Zn Mn Zn Mn Influent PLB effl. Tray Chan Diluted Infl, Tray PLB effl Chan pass Pass process Pass PLB recycle Chan Alkalinity and metal concentrations reported in mg/l. that it would exceed the acidity of the water. After 200 min of total process time, the effluent water had built up to an alkalinity of 720 mg/l. This water was air stripped, then recirculated through the limestone channel for 2 h, with a resulting 98% removal of Zn and 42% removal of Mn (Table 5; Figure 7, dashed line). In this case, the Ca 2 concentration decreased over the course of the test, indicating that calcite precipitation was occurring. This was the most promising result for the Palmerton water, and suggests that further optimization, such as increasing the limestone channel length and seed addition, may result in improved metal removal. Further process testing was not possible at this point. Should additional resources become available, the effects of water blending, and reprocessing through the PLB and the channel coprecipitation setup should be investigated with the objective of increasing Mn removal, and developing a complete treatment process for the Palmerton water. It was not possible to effectively determine sludge volume for the coprecipitation process with the synthetic solution, but sufficient quantities of precipitate were generated in the testing of the Palmerton water so that precipitate volume and characterization testing was possible. Results from the PLB recycle tests described above showed that settled sludge volume after air stripping was 19 9 ml/l (n 3), much less than the 130 ml/l that was found for ph adjustment of the Palmerton water. Analysis of the dried precipitate by X-ray diffraction

13 REMOVAL OF ZINC AND MANGANESE FROM MINING WATERS 893 Figure 7. Precipitation kinetics for Palmerton water channel tests. showed that the precipitate contained calcite and hydrozincite, Zn 5 (OH) 6 (CO 3 ) 2. Previous investigations of Zn precipitation in the presence of calcite have found similar compounds (Zachara et al., 1989). The limestone from the channel that had been exposed to the Palmerton water was also analyzed to characterize the nature of the precipitated phases. An X-ray fluorescence scan of the raw and exposed limestone samples showed that both Zn and Mn were present in the exposed limestone, but not in the raw limestone. Selected limestone particles were also examined by scanning electron microscopy (SEM) coupled with energy dispersive X-ray microprobe analysis. Particles with high Zn content were noted on the limestone surface, probably hydrozincite carried over from the air strip section. However, no discrete particles containing Mn were observed on the limestone surface. In confirmation, X-ray diffraction results of the exposed limestone did not show any Mn phases, and in fact, did not differ significantly from calcite. These results suggest that the Mn was being taken up in the calcite as a solid solution, as has been found previously (Lorens, 1981; Pingatore et al., 1988). As a crystalline phase, the Mn-bearing calcite would be expected to be much less voluminous than the sludges obtained by ph adjustment. Some of the species responsible for excessive sludge volume upon ph adjustment are gypsum (CaSO 4 2H 2 O) and magnesium oxides such as brucite (Mg(OH) 2 ). The limestone used in the Palmerton channel tests was also analyzed for MgO and total sulfur (S) to determine whether these compounds had been formed. The MgO content was 0.62% for the fresh limestone, while the channel limestone showed 0.63 and 0.61% MgO in replicate analysis. Similarly, the fresh limestone assayed 0.06% S and the channel limestone gave 0.07 and 0.10% S in replicate. Thus, there is scant evidence for the formation of gypsum or brucite in the limestone channel which would lead to excessive sludge volume. Overall, these results suggest that the coprecipitation process would result in much lower sludge volumes than ph adjustment, which would lead to significant cost savings. SUMMARY AND RECOMMENDATIONS A novel CCP process was tested for the removal of Zn and Mn from mining impacted waters. Optimum condi- ENVIRON ENG SCI, VOL. 24, NO. 7, 2007

14 894 tions for saturation of water using the PLB system were (1) flow rate of 0.25 L/min, (2) CO 2 pressure of 103 kpa, and (3) carbonator water level of 25 cm or less. Following treatment in the PLB system, a synthetic mine water containing 50 mg/l each of Mn and Zn was processed by three different methods to implement the CCP process. The limestone channel configuration was found to give the best results, at about 95% removal of both metals after 2 h of contact. The limestone channel configuration also had the greatest ratio of calcite seed/water and demonstrated the importance of the seed material in the Mn removal process. Actual mining impacted water from the Palmerton superfund site was also tested, with a resulting nearly complete removal of Zn and 40% removal of Mn using the PLB/limestone channel configuration. Additional testing would be required for complete treatment of the Palmerton water. Suggested approaches for complete treatment would include blending of the test water with other less-impacted waters to reduce the metal load to be treated. Multiple passes through the PLB and limestone channel should then be able to decrease Mn to required discharge concentrations. Additional variables affecting the performance of the limestone channel may also be important and should be tested, including the effect of channel grade, length, limestone loading, and recirculation rate. However, for water with lower metal concentrations, tests indicate that the CCP process offers a substantial improvement in cost efficiency over the traditional ph adjustment method, because of the lower reagent cost and sludge volume, the selectiveness of the process to the target metals, and the elimination of the need for ph readjustment. REFERENCES APHA (AMERICAN PUBLIC HEALTH ASSOCIATION). (1995). Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC. Author. 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