CHARACTERIZATION AND LEACHING OF HEAVY METALS FROM MUNICIPAL SOLID WASTE INCINERATOR FLY-ASHES

Célia Ferreira ¹, Susana Llamas ², Manuel F. Almeida ³

¹ Escola Superior Agrária de Coimbra, Sector de Química 3040-316 Bencanta, Portugal, e‑mail: celia@mail.esac.pt

² Facultad de Ingeniería, Universidad Nacional de Cuyo, C.C: 405, Mendoza, Argentina

³ Faculdade de Engenharia da Universidade do Porto, Rua dos Bragas, 4099 Porto Codex, Portugal

* To whom all correspondence should be sent

ABSTRACT

Fly ashes from two Municipal Solid Waste (MSW) Incinerators were studied regarding heavy metals composition, their availability for extracting and mobility. A sequential extraction with an acid solution was attempted and pH and conductivity values of solutions were registered. Also, in the eluates Mn, Ni, Fe, Cu, Zn, Pb and Cd were determined using Absorption Atomic Spectrophotometry (AAS). This study is a step in a research whose goal is obtaining a less toxic residue that could be used instead of being landfilled.

INTRODUCTION

A MSW incineration facility started to work recently in Porto, Portugal, and fly ash generated is landfilled after a stabilization/solidification process. This is a usual practice for this kind of residue, whose high content in heavy metals is one of the main issues conditioning its utilization. Detoxifying this residue may be a good approach to open some potential alternatives of reutilization, thus avoiding deposition in landfills and subsequent problems

This work intended to acquire a better knowledge about fly ash properties and behavior, namely its leaching characteristics, in order to proceed towards better routes of treatment.

The maximum amount of heavy metals that could be released to the environment during the lifetime of fly ashes was evaluated through a so-called Availability Test. Since the availability test is a worst case scenario and does not represent real conditions in the field, another test, NEN 7343, was performed to evaluate the mobility of heavy metals in a more realistic way. Finally, some leaching procedures to extract heavy metals with an acidic solution were also tested. One of the objectives of this step is to find the basis of minimizing water consumption while removing as much metals as possible. Thus, different liquid:solid (L/S) ratios were tested, ranging from 1.3 to 9.0 for one of the samples and 0.2 to 1.5 for another. The possibility of improving heavy metals extraction using low L/S ratios was also tested by acidification and recirculating the leachate several times.

METHODS

Two samples were tested: (i) sample A, from the incineration facility in Porto; and, (ii) sample B, from an existing facility in UK, that uses the same incineration and off‑gas treatment processes. Quantitative chemical analysis of sample B fly ash was carried out according to a SW-846 USEPA method (USEPA,1997). With this objective approximately 0.5 g sample was digested for 15 minutes in a microwave oven with 10 ml concentrated nitric acid, 2 ml concentrated hydrochloric acid and 2 ml deionised water. The suspension was cooled to room temperature and vacuum-filtered using a 0.45 µm membrane. The filtrate was diluted to 100 ml and some heavy metals were determined by AAS using a Unicam Solaar32 AA, System, mod. 969.

An Availability Test was carried out on sample B through two consecutive extractions, the first at constant pH 7 and L/S ratio of 50 and the second extraction at constant pH 4 and L/S ratio of 50. At each extraction pH was maintained constant by adding HNO₃ 1:1. Cumulative L/S ratio was 100. Following, some heavy metals were determined by AAS in the final solution.

NEN 7343 (1995) protocol was the basis of the mobility test carried out with both samples A and B. In such column test seven fractions are collected and heavy metals determined by AAS. Cumulative L/S ratio in this method is 10.

The leaching experiments were carried out using two acrylic columns with h=30.0 cm and Ø_int=5.2 cm operated in upflow using a peristaltic pump, Masterflex mod.7018.20 for sample A and Ecoline VC-360 for sample B. Fly ashes samples were carefully loaded into the columns to ensure good packing. The columns were weighted before and after filling, to determine the total mass sample, and the values corrected for moisture content (determined in sub-samples dried at 105 ± 5 ºC).

The extraction procedure used for sample A is described next. In a first stage deionised water acidified with HNO₃ to pH = 4.0 and conductivity 4.84 µS/cm was passed through the fly-ash bed in column 1 and the leachate obtained, referred as C1.E1 (Column 1, Extract 1), was collected in a flask. In a second stage a fresh leaching solution with pH 4.0 passed through the same column and respective leachate is referred as C1.E2. For both extractions, precipitates were formed in the collection flasks.

A column 2 with fresh A fly ashes was percolated with C1.E1 solution and the leachate collected is referred as C2.E1. Following, by passing C1.E2 extract through column 2, a new leachate was obtained. This leachate was collected in the same flask than C2.E1, and the final leachate is referred to as C2.E2.

The leaching procedure for sample B was slightly different, since deionised water acidified with HNO₃ to pH = 4.0 leached fly ash in column 3 and leachate was reintroduced into the column for several times. This test was carried out at room temperature with a L/S ratio of 1.5 and 8 h of contact time.

Fly ashes in column 4 were subjected to a series of two sequential extractions. The first extraction used the leachate from column 3. This solution was re‑introduced continually into column 4, in a closed circuit loop, and acidified to pH=2 (with HNO₃ 1:1) before each passage. Whenever acidification originated the formation of a precipitate the solution was filtered through medium size pore filter paper. L/S ratio for this extraction was 0.2 and the contact time 4 days.

The second extraction of column 4 used deionised water acidified with HNO₃ to pH = 4.0. As previously, the solution was used in a closed circuit loop and was acidified to pH=2 before each new passage. Filtration was not required. The contact time was 3 days and L/S=0.3, thereby producing a cumulative L/S ratio of 0.5.

The pH and conductivity were registered for all the experiments and an aliquot sample of each extract was taken and preserved for posterior analysis according to APHA (1998) methods.

The concentration of heavy metals in the extracts was determined by AAS using air-acetylene flame.

RESULTS AND DISCUSSION

Table 1 compares the results of the quantitative chemical analysis of fly ashes B and the availability test. Zinc, iron and lead are the major constituents with concentrations of respectively about 12, 8 and 5 g/kg of fly ashes, and together represent almost 95% of the determined metals. All other metals' concentrations are less than 1 g/kg of fly ashes. The leachability depends on the species considered, ranging from 0.2% for iron to almost 88% for cadmium. Table 2 and Figure 1 present the results obtained for NEN 7343 test for samples A and B.

Table 1. Composition and availability for leaching of heavy metals in fly ash B

	Mn	Ni	Fe	Cu	Zn	Pb	Cd
Composition (mg/kg)	442	147	8389	631	11825	5516	222
Availability (mg/kg)	94	30	19	296	8389	777	195
% available	21.3	20.4	0.2	46.9	70.9	14.1	87.8

Table 2. Heavy metals leached from fly-ashes A and B, (mg/kg)

	Mn	Ni	Fe	Cu	Zn	Pb	Cd
Sample A
total	0.49	1.47	12.92	6.27	21.42	11.73	3.50
Sample B
total	0.09	2.71	1.61	0.08	1.85	12.52	3.40
mobility (‰)	0.96	90.3	84.7	0.27	0.22	16.11	17.4

The main difference in the fly-ashes is that sample A leached Fe, Cu and Zn several times more than sample B. For Pb, Cd, Ni and Mn differences are not meaningful. Mobility, expressed as the ratio of mass of metal leached to mass available for leaching x 1000, was calculated for sample B. Table 2 shows that mobility of these metals is very low, which means that in field conditions, as simulated by NEN, most of heavy metals will be retained in fly-ash and are not easily released to the environment.

The extraction experiments gave the values for pH and conductivity in Table 3.

Table 3. pH and conductivity in different extracts

Column	extraction	L/S	pH	Conductivity
		[l/kg]		[mS/cm]
Sample A
C1	E1	5.0	12.24	102.7
C1	E2	4.0	12.52	8.5
C2	E1	1.5	11.90	171.4
C2	E2	3.5	11.60	78.6
Sample B
C3	E1	1.5	12.24	71.4
C4	E1	0.2	11.37	189.7
C4	E2	0.3	10.97	144.5

It can be seen from Table 3 that the pH of all leachates is above 10 through all the extractions for both samples.

The analysis of conductivity values leads to the following considerations: firstly, the conductivity increases as the solution goes from the first column to the second column (from 102.70 mS/cm to 171.40 for sample A; and, from 71.4 to 189.7 for sample B), indicating an increase in dissolved salts. This could mean that the leachate of the first column, when put into contact with new fly ash, is still able to dissolve more materials.

Secondly, when a new solution is used (E2) in column 1 the conductivity of the leachate is very low. In other words, all easily soluble materials were removed in the first extraction and dissolution of new materials from fly ash is very difficult. This means that in sample A a good extraction is obtained for L/S=5.

Finally, when a new solution (E2) is used in column 4 (cumulative L/S=0.5), the conductivity decreases circa 30% but remains high. A more complete extraction for this sample would require higher L/S values, probably around 5 l/kg, as obtained for sample A.

The analytical results concerning heavy metals content in the extracts are shown in Tables 4 (sample A) and 5 (sample B). The results show that for both samples zinc and lead are the principal leached metals, representing more than 88% of the total metals extracted. These results are consistent with those obtained in the availability for leaching test (Table 1) where these two metals represent almost 94% of all leachable metals.

Table 4. Heavy metals content of sample A extracts (mg/L)

	Mn	Ni	Fe	Cu	Zn	Pb	Cd
C.1 – E.1	0.04	0.03	1.64	5.01	30.52	128.37	0.63
C.1 – E.2	0.15	0.04	5.11	0.09	71.82	189.83	1.01
C.2 – E.1	0	0	0	0	0.83	7.55	0.02
C.2 – E.2	0	0	0	0.08	6.47	9.27	0.06

Table 5. Heavy metals content of sample B extracts (mg/L)

	Mn	Ni	Fe	Cu	Zn	Pb	Cd
C.3 – E.1	0	0.67	2.57	0.22	11.32	29.8	0.62
C.4 – E.1	0.23	1.58	0.6	0.26	12.03	8	0.09
C.4 – E.2	0.22	1.24	0.17	0.07	8.64	5.35

The concentration of the analyzed metals does not follow the same pattern of conductivity. In fact, for sample A, as extract 1 goes from column 1 to column 2 the conductivity increases and metals concentration decreases. Sample B presented a somewhat different behavior since Mn, Ni, Cu and Zn concentration increased, as expected, and decreased for Fe, Pb and Cd. This decrease in metal concentration in columns 2 and 4 could have two explanations: the metals precipitated either in the collection flask or inside the column.

The second extraction for sample A (E2) removed even more metals than extract 1 (E1), except for copper. This does not happen for sample B, in which each extract was recirculated several times inside the column. This seems to indicate that multiple passages of the same solution after adjusting pH lead to a better removal than a single passage. Even though for sample B concentrations for extract 2 (E2) are lower than for E1, the values are still high, suggesting the need for a higher L/S ratio to improve extraction or the use of a different leaching solution.

CONCLUSION

It was possible to increase the concentration of Mn, Ni, Cu and Zn in sample B by recirculating the same solution after being acidified. However, the increase was not as high as could be expected and for some metals (Fe, Pb and Cd) the concentration decreased, as also happened in sample A. This could be explained by the occurrence of precipitation or/and complexation reactions. A better understanding of the mechanisms that control such a complex phenomena requires further investigation.

REFERENCES

USEPA (1997), Test Methods for Evaluating Solid Waste, Physical/Chemical Methods (SW-846), CD-ROM v.2, EPA/NTIS.

APHA (1998), Standard Methods for the Examination of Water and Wastewater, American Public Health Association‑American Water Works Association‑Water Environment Federation, 20th. Edition.

NEN 7343:1995 Leaching characteristics of solid earthy and stony building and waste materials. Leaching tests. Determination of the leaching of inorganic components from granular materials with the column test. The Netherlands, Feb. 1995.