Release of metals from MSW I fly ash and availability in alkali condition

Waste Management, Vol. 16, Nos 5/6, pp. 537-544, 1996 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0956-053X/96 $15.00 + 0.00

Pergamon PII: S0956-053X(96)00095-5

RELEASE OF METALS F R O M M S W I FLY ASH AND AVAILABILITY IN ALKALI C O N D I T I O N

Satoshi Mizutani, Tsuneyuki Yoshida, Shin-ichi Sakai and Hiroshi Takatsuki Kyoto University Environment Preservation Center, Yoshida-honmachi, Sakyo-ku, Kyoto-city, Kyoto 606-01, Japan

ABSTRACT. The release and availability of Pb, Cd, Cu, Zn from municipal solid waste (MSW) fly ash were studied with an

emphasis on its behavior under alkaline conditions. Serial batch tests using acidic and alkaline solutions as leachant and a pH stat test were carried out. From the results of serial batch test: (1) Even if the ash comes into contact with strong acid for a long period, pH of the leachates from some fly ashes are high, and the concentration of Pb in the leachate is high. (2) When the ash comes into contact with the alkaline leachant, even if the fly ash has little alkalinity, the pH of the leachate soon increases and the concentration of the Pb and Zn in it are high. (3) The fact that leaching of heavy metals from fly ash depends on the pH of the leachate was certificated. From the pH stat test: (1) The larger the liquid per solid ratio (L/S), the more metals are released. (2) Leaching behavior of Pb is different from other metals and leached in high concentration in alkali condition. (3) Concentration of metals in leachate is controlled not only by solubility but other factors and were certificated. Moreover, considering the properties of the ashes, the optional procedure of the availability test for Pb was proposed. © 1997 Elsevier Science Ltd

dry exhaust gas treatment system maintain high pH for a long time, 9 it seemed the improper condition to maintain pH 4 of availability test for those ashes. Therefore, several MSW incineration fly ashes in Japan were subjected to serial batch tests using acidic and alkaline solution, and the changes of pH and concentration of heavy metals in leachate were investigated. In addition to changing the L/S in maintaining the pH of leachate, the change of heavy metal leaching quantities was investigated.

INTRODUCTION Municipal solid waste (MSW) incineration fly ash is a granular material that contains many hazardous constituents among which are heavy metals. Therefore, when it comes into contact with water, hazardous constituents can be leached. Because of such properties, it is one of the most important wastes for which the leaching test is applied, and a number of studies on leaching tests using the fly ash as samples are reported.l-5 JLT-13, the leaching test in Japan, is a single batch test used in regulatory frame work. However, it has been stated to be almost impossible to evaluate the behavior of waste in a final landfill site over a long period using a single batch test. 6,7 Recently, the International Ash Working Group (IAWG), a group studying the disposal and treatment of incineration ashes, proposed the concept of availability and the availability test. Availability is defined as the maximum leachability that can be considered with the worst case in the final waste land disposal site. We tried to apply this test to some ashes. However, especially in alkali condition, there are some ashes from which Pb leaches out more than the availability obtained using the availability test. 8,9 Because some fly ashes from the

EXPERIMENT Samples

MSW incineration fly ashes in Japan were used. Considering the differences in the types of incinerators, dust collectors and exhaust gas treatment systems, six fly ashes were selected for the study. In Japan, most incinerators collect fly ash and air pollution control (APC) residues at the same time and a regulation in "The Waste Disposal and Public Cleansing Law" demands treatment of both of them. Therefore, we use the term "fly ash" for these sampies. They are a mixture of fly ash and APC residues according to the definition by the IAWG. The origin and the contents of metals are shown in Table 1. 537

S. M I Z U T A N I E T ,4L.

538

TABLE 1 Origin and Contents of Ashes Ash

A B C D E

Incinerator

Stoker Stoker Stoker Stoker Fluidizedbed

Dustcollector* APC

EP BF EP EP EP

Dry Dry Wet Wet Semi-dry

Contents (mg/kg) Ca

Cd

Cu

Pb

Zn

210,000 370,000 120,000 105,000 330,000

74 76 410 109 27

570 410 2600 500 1900

1400 1900 10,000 2900 1300

6000 5500 29,000 12,900 2200

*EP= Electronprecipitator; BF= Bug house.

Procedure Experiment 1. Serial batch test using acidic and alkaline leaching solution. Purpose. pH change and pH dependent leaching behavior of heavy metals. Leachant 1. acidic leaching solution: nitric acid (0.056 mol/L); pH 1.3 (HNO3 (4.3 mL) dilutes to 1 L); 2. alkaline leaching solution: slaked lime solution; pH 12.0. (Ca(OH)2 (0.50 g) is dissolved in distilled water up to 1 L).

were selected) and 800 mL of leachant were added to a 1000 mL glass beaker. Stirring by a magnetic stirrer, we determined and maintained the pH by adding pH adjusting agents. After stirring for 6 h, the quantity of adjusting agents added is noted. After settling for 30 min, the leachate was filtered using a membrane filter (0.45 ~tm). Filtrates were analyzed by ICP spectrometry. The determined concentrations were exchanged for "release (mg/kg)" using the final leachate quantity, which is the total of the initial leachant and pH adjustment agents. We use this release in the following discussion. RESULTS AND DISCUSSION

Types of test. Serial batch test (L/S = 10x 10 = 100).

Experiment 1

Methods. Serial batch test was carried out in closed 1000 mL glass bottles, with 50 g of samples and 500 mL of leachant added. After two hours of shaking, the leachate was filtrated using a membrane filter (with 0.451am of pore size) and the filtrate was called Sample 1. The residue with 500 mL of leachant was subjected to the second step. It was added to the glass bottle with the leachant again and agitated for two hours. After that we obtained Sample 2 by the same operation. This was repeated 10 times (final L/S = 100). As for each filtrate, after determining the pH, the concentration of target metals was analyzed by inductively coupled plasma (ICP) spectrometry.

Leaching behavior of the metals from the fly ash in contact with the acidic leachant

Target metals. Pb, Cd, Cu, Zn. Experiment 2. pH stat test at different L/S values. Purpose. Determination effect of L/S on the leached quantity of metals under alkaline and acidic conditions. Leachant. Distilled water, pH adjusting agents: IN HNO3, 5N HNO3, 1N NaOH, 5N NaOH. Types of test. Extract test using pH controller. Method. Definite quantities of fly ash (in order to change L/S, 80 g, 26.7 g, 8.0 g, 2.67 g, 0.80 g, 0.27 g

pH. In the results of Experiment 1, the final pH plots versus L/S in the experiment that leachant is 0.05 mol/L HNO3 are shown in Fig. 1. For the acidic leachant, there are many types of change in the pH of the leachate. Various kinds of types of behavior exists. In B-fly ash, which has high acid neutralizing capacity, the pH continued to be over 12.4 until L/S=70, and showed alkaline (pH 8) at L/S= 100. Although the initial leachates of E- and A-fly ash showed high pH (12.3-12.6), which are the same levels as B-fly ash, at L/S = 100 the final pH figures were 6.1 and 3.2, which are the same level of "neutral fly ash" (the ash whose initial pH shows the neutral or weak alkaline, C- and D-fly ash in this study.) As for the initial pH of leachate, because pH of saturated solution of Ca(OH)2 is approximately 12.3, if the 100% of Ca(OH)2 in fly ash leached out, at least 1.5 g of Ca(OH)2 are contained in the 100 g of the fly ash, the leachate shows pH 12.3. Fly ash contains 10-37% of Ca (Table 1). Though all Ca are not derived from Ca(OH)2, so long as Ca contents vary widely, it is almost impossible to estimate the final pH and acid neutralizing capacity based on only initial pH.

539

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Leaching concentration and the release. Leaching concentration of Pb and Zn in leachate at serial batch test versus L/S are shown in Fig. 2(a) and Cd and Cu are shown in Fig. 2(b). From B-fly ash, Cd and Cu did not leach out at all. Although there seems to be no relationship between L/S and the leaching concentration, it becomes understandable in the light of the pH of leachate and leachable quantity. For A-fly ash, leaching starts to increase due to the mobilization of Pb under acidic conditions until depletion of Pb is reached. At this point all of the Pb availability for leaching has been leached out. In contrast, for B-fly ash, Pb leached in high concentration at first, and then leaching concentration decreased bit by bit because the pH were maintained to be high, and the depletion of leachable Pb under alkaline condition is reached. As for other fly ashes, these phenomena can be explained by the pH and the depletion of the availability for species. Next, the cumulative release from A-fly ash and B-fly ash for metals is shown in Fig. 3(a),(b). The availability extracted based on the availability test is also shown in the same figure as a dotted line. As for A-fly ash, the cumulative release of the metals approached the availability. It is clear that the cause of decrease of the leaching concentration for these metals is depletion of the availability in the fly ash. The final leaching quantity is higher than the availability for Pb and Cu, because the final pH is lower than pH 4. On the other hand, the Pb and Zn leaching quantity from B-fly ash is interesting. The leaching quantity of Zn is lower and the Pb is higher than the availability. This is the result of the solubility of the Zn and Pb at alkali condition. In the case where the fly ash maintains alkaline pH like B-fly ash, the leaching quantity extracted by the experiments and the availability based on the availability test are different. Leaching behavior of metals from fly ash in contact with alkaline leachant. For fly ash, the most important leachant at final land disposal sites is rainfall. Because of the contents of the CO2, SOx and NOx in

the atmosphere, rainwater is basically acidic solution. Moreover, organic acid is generated by bacteria using raw refuse or sewage sludge as nutrition. The leachant selection is made such as to reflect these anticipated conditions but recently many fly ashes and bottom ashes that have a high acid neutralizing capacity are generated from dry exhaust gas treatment system to reduce acid gas emission. Considering the conditions at land disposal sites in Japan, the combined disposal of ash with raw refuse is not carried out, and most ways of disposal are monofill in which fly ash and bottom ash are codisposed. Taking this fact, and the fact that the waste with a high acid neutralizing capacity is increasing, into account, at the final land disposal site there is a great possibility that landfilled fly ash is contacted with alkaline leachate derived from the other fly ash. Therefore, having not been reported so much, we applied the serial batch test using alkaline leachant to neutral fly ash.

pH. For C- and P-fly ash, the change of pH of leachate in the seri~tl batch test using alkaline leachant is shown in Fig. 4. There is a large difference between the two neutral fly ashes. Leachate pH of D-fly ash increased rapidly when it came in contact with alkaline leachant. On the other hand, C-fly ash showed a low pH (< 12) until L/S=40. These differences seemed to be derived from the availability of metals of the two fly ashes. Metals which have + 2 charge precipitates as such: M 2+ -t- Ca(OH)2 ~ M(OH) 2 ~ + Ca 2+ (M • Cd, Cu, Zn, Pb etc.) And amphoteric metals such as Pb and Zn form complex ion and solute: Pb 2+ + 3OH- ~ Pb(OH)~, Zn 2+ + 3OH- ~ Zn(OH)3 Therefore, it is expected that there is a relationship between the alkaline neutralizing capacity and the total metal leaching quantities under alkaline condition. The quantitative analysis will be needed.

Leaching concentration and release. The Leaching concentration of metals versus L/S are shown in Fig. 5. As pH increases and goes beyond pH 11, the leaching concentration of Pb increases rapidly. It has been pointed out that Pb from APC residue shows the maximum leachability in the alkali condition, 7 but the behavior of neutral fly ash in alkali condition should also be paid attention to. Leaching concentration and pH of the leachate. As described in the previous sections, because the composition is variable for ashes, it is difficult to estimate when the heavy metals leach in high concentrations

S. M I Z U T A N I E T A L

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RELEASE OF METALS FROM MSWI FLY ASH

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(b) FIGURE 3. (a) Release and availability of metals (A-fly ash). (b) Release and availability of metals (B-fly ash),

that may cause harmful effects to the environment by watching only L/S. On the results from serial batch test, the leaching concentration of metals to pH of leachate is shown in Fig. 6. These figures show the relationships only

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between the pH and leaching concentration of the leachate, and do not show when the metals leached out. It shows that leaching in high concentration of the metals (Pb, Cd, Cu, Zn) is explicable by pH alone. In Fig. 6, waste disposal standards for Pb and Cd in Japan (0.3 mg/L) are also shown. The low concentrations for Cd at low pH are the result of the depletion of leachable Cd. From these figures, if we regard Pb and Cd as hazardous metals in the leachate from MSW fly ash, the dangerous areas of leachate are pH > ll and pH <4.5 for Pb, and pH <7 for Cd. Therefore, at pH levels over 11 and under 7 where the risks of waste are expected to become very high, it is necessary to research the leaching behavior by applying the leaching test. If leaching concentrations in these areas are low, that waste may be regarded as non-hazardous as far as Pb and Cd are concerned.

542

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FIGURE 5. Release of Pb, Zn and pH from neutralfly ash. 1000

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FIGURE 6. Leachingconcentrationand pH. Next, we think about L/S. The most important leachant that has the possibility of coming into contact at landfill sites is rainfall. Annual precipitation in Japan is about 1500 to 2000 mm. We assume the fly ash is deposited in 10 m at final land disposal sites and its density is 1 g/cm 3. At this time, L / S = 100 means a period of 500 to 700 years. On the other hand, because the L/S in the serial batch test or column test and the L/S in the single batch test are able to be considered the same scale, 6,1° L/S = 100 in serial batch test also shows about 500 years. As for the period, it seems to be long enough to evaluate leaching behavior. N o w we attend to Fig. 1 again. Alkali fly ashes can be classified into two types according to the final (at L/S = 100) pH. The final pH is > 7 (B-fly ash) and < 7 (A- and E-fly ash). B-fly ash maintains

pH > 12 up to L/S = 80, and it is in neutral area at L/S = 100. Therefore, from hazards leaching from B-fly ash in the real environment, the behavior in alkali condition is more important than in acidic condition. On the contrary, for A- or E-fly ash, though their initial pH is very high, the final pH is very low. Therefore, it is important to investigate the leaching behavior in both acidic and alkali conditions. Experiment 2 In the former section, the serial batch test using acidic and alkaline leachant was applied, and the change of pH of leachate and the leaching of heavy metals from fly ashes was researched. Next, in order to investigate the effects of the availability, L/S and pH of the leachate, each fly ash was maintained at

RELEASE OF METALS FROM MSWI FLY ASH

543

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FIGURE 7. (a) Cd releaseverusL/S (B-flyash). (b) Cu releaseversus L/S (B-flyash). (c) Zn releaseversus L/S (B-flyash). (d) Pb release versus L/S (B-flyash). pH 12.3, 4, 2, in different L/S, and the leaching behavior and maximum leaching ability of fly ash was observed. Samples are four ashes (A, B, C, E) out of five fly ashes used in Experiment 1 and the L/S changed from 10 to 3000 in six steps.

Change of pH and the release. As for Cd, Cu, Zn, Pb from B-fly ash, the leaching quantity versus L/S is shown in Fig. 7. Cd does not leach at all at pH 12.3. At pH 4 or 2 the leaching quantity was the same level. Behavior of Cu is similar to Cd. At pH 12.3 it shows no leaching. Focusing on leaching quantity in acidic condition, the leaching quantity at pH 4 increases rapidly with the increasing of L/S and the leaching quantity at L/S = 3000 was about 1.5 times as much as L/S = 100. Therefore, it does not show the maximum leachability, For Zn, Cd and Cu at pH 12.3 the leaching quantity was less than in acidic condition. Furthermore, in acidic condition the effect of L/S was small but at pH 12.3 the leaching quantity increases rapidly with an increase of L/S. At L/S = 100 leaching quantity in alkali condition is low. Comparing the leaching quantity at pH 4 with 12.3 there is a difference about 15 to 50 times. As for Pb, the leaching quantity is independent of L/S at pH 2 being higher than at pH 4, 12.3. Especially at L/S = 100 and 300 there is three to 10 times difference. Compared at pH 4 with pH 12.3, leaching

quantity in alkali condition is higher at low L/S, and at L/S = 1000 and 3000 the leaching quantity in acidic condition is higher. At pH 2, 12.3, when L/S is 300, 1000 and 3000 the leaching quantity showed the same concentrations. But leaching quantity of both samples shows differences twice. We were not able to obtain the availability in alkali condition. On the other hand, at L/S---100, comparing the leaching quantity at pH 4 with 12.3, only Pb leached more at pH 12.3. Although the current availability test consists of the two steps that extracts pH 7 and 4, only for Pb leaching quantity, the step to determine the behavior in the alkali condition should be made as one optional procedure.

Change of L/S and availability. As a result of pH stat test, the leaching quantity of Pb and Cu tends to increase with increasing L/S. As for Cd and Zn, the leaching quantity does not increase because the depletion of the leachable constituents has occurred at even low L/S. Here, in order to research the change of leaching quantity and availability at large L/S, a mixture of pure PbSO4, Na2SO4 and NaC1 instead of fly ash was subjected to the pH stat test similar to Experiment 2 (Fig. 8). This result apparently shows that there is a leaching quantity increase with increasing L/S at every pH. Although there are differences of leaching quantity at low L/S between

544

S. MIZUTANI ET AL. evaluated at the first step. The leachant is Ca(OH)2 saturated solution, which is the most p o p u l a r alkaline solution at the final disposal site, and L/S is settled at 100. Second and third steps are applied to the current availability test (extraction at p H 7 and 4 at L/S = 50). The leaching concentration o f each leachate should be determined respectively and the total leaching quantity should be calculated finally.

100000 Contents

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t0oo pH4 r "

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different pH, by m a k i n g L/S large, they a p p r o a c h each other, and finally close to their contents. W h e n the leaching o f heavy metals is controlled only by chemical equilibrium o f solubility, if ash comes in contact with an infinite quantity o f leachant the metals dissolve completely, in theory, and leaching quantities are independent o f the p H o f the leachant and should show the same value. (Of course, the quantity o f leachate is controlled by the detection limits o f metals in fact.) In Fig. 7, however, the difference o f p H generates the difference o f the final leaching quantity. This means leaching behaviors o f metals are affected by not only solution equilibrium but also other factors.

Proposal for the optional procedure of the availability test to determine Pb availability. Availability estimates the final metal leaching quantity in weight per unit o f waste in the worst case at final disposal. The worst case scenarios suppose that the waste are g r o u n d and contacted with strong acid for a long time. In order to determine the availability, I A W G propose the availability test as part o f the overall evaluation o f materials. F r o m the results o f our two experiments, however, the current availability test m a y underestimate in the case o f Pb for two reasons. One concern is leachant and pH. A t the final disposal landfill site, it is possible for the ash to come in contact with strong alkaline leachant, and as for Pb, the alkaline solution has a stronger extraction ability than acid in certain condition. The other problem is L/S. The final L/S in current availability tests is 100. As for Pb and Cu, taking the results o f Experiment 2 into a c c o u n t L/S = 100 is not a large e n o u g h leachant quantity. In the optional procedure o f the availability test, the leaching o f Pb in alkali condition should be

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