Remediation of polluted soils by utilizing hydrothermally treated calcareous fly ashes

CHINA PARTICUOLOGY Vol. 4, No. 2, 65-69, 2006

REMEDIATION OF POLLUTED SOILS BY UTILIZING HYDROTHERMALLY TREATED CALCAREOUS FLY ASHES A. Moutsatsou* and V. Protonotarios Laboratory of Inorganic and Analytical Chemistry, Department of Chemical Engineering, National Technical University of Athens, Greece *Author to whom correspondence should be addressed. E-mail: [email protected]

This paper investigates a treated fly ash to act as a synthetic zeolite to remediate soils polluted with heavy metals and metalloids (As, Pb, Cu, Zn, Fe, Cd and Mn). Four types of such ‘zeolites’ were synthesized by hydrothermal treatment of a calcareous fly ash derived from Greek lignite-fired power plants: two with excess of sodium hydroxide in a -1 -1 solid/liquid ratio of 50 g⋅L , and two with excess of fly ash in a solid/liquid ratio of 100g⋅L . Soil samples were obtained from a former mining site at Lavrion, Greece. Mobilization and transfer of metals to the retention agents was effected by using HCl aq 1M, with satisfactory results with respect to As, Pb, Cu, Mn and Cd. The great variety of metal complexes in soil was found to be of major importance for the effectiveness of the overall process. The final products were solidified either on their own, or by using additives such as lime and cement.

Abstract

Keywords

fly ash, zeolites, soil washing, metals, metalloids, retention, stabilization, solidification

1. Introduction Uncontrolled disposal of waste materials has resulted in significant metal contamination in soils (Davis & Singh, 1995). Metals and metalloids such as Pb, As, Cu, Zn, Cd and Mn have been recognized as an important hazard, both for public health and the ecosystem (Alam et al., 2001; Kedziorek and Bourg, 1999). These substances, when present in soils, are not biodegradable by natural process and remain in the ecosystem (Nael et al., 1997; Kedziorek & Bourg, 1999). Remediation technologies for contaminated soil include stabilization/solidification, soil flushing and washing, electrokinetic methods, bioremediation and phytoremediaton techniques, thermal treatment, etc (Mulligan et al., 2001). A new approach utilizes the high adsorptive capacity of certain industrial by-products (like fly ashes and slags) for metals and other toxic elements (Vlyssides et al., 2004). A permanent solution for soil pollution abatement could be the removal of contaminants from the soil in order to minimize environmental and health risks. Chemical extraction followed by retention and stabilization of pollutants on specific substrates represents a promising method for that purpose (Vlyssides et al., 2004). At the Lavrion Technology and Cultural Park (LTCP) located at the metallurgy complex of the former “Compagnie Francaise des Mines du Laurium”, about 55 km from Athens, mining operations for silver and lead started as early as 3 000 BC but ended in 1980 (Konofagos, 1980). The intensive mining and metallurgical development at LTCP, has resulted in serious contamination problems affecting the local community in forms of slags, sulfur compounds, smelting waste, etc, all rich in hazardous metals and metalloids. The current study deals with the mobilization of metals and metalloids from soil samples obtained from LTCP and their retention on zeolites synthesized from Greek fly ashes. The final step of the process includes the stabiliza-

tion/solidification of the soil-zeolite products, either by exploiting the pozolanic attributes of the fly ashes or by utilizing additives such as lime and cement.

2. Materials and Methods 2.1 Soil samples Surface samples were collected from the topsoil (0-0.25 m) to investigate the extraction/retention/stabilization of their metal content under the effect of a selected chemical reagent (HCl 1 M). Soil pollution, as shown in Table 1, consists mainly of slags, mixed with sulphur compounds waste and low-grade lead metallurgical condensates, all by-products of mining and metallurgical activities in the past. Along with the actual metal-metalloid concentrations, determination of additional physicochemical and mineralogical characteristics of the soil was conducted according to standards: pH (ISO 6588), chloride content (EN 196-21), specific gravity (ASTM C642-90) (Table 1) and bioavailable fractions of pollutants in soil (Table 2). Furthermore, X-Ray Diffraction Analysis (Siemens D-500) and Thermogravimetric Analysis (Mettler C TGA/STDA 851 ) were also conducted for purposes of identifying the structure of pollutant compounds (Table 3). Table 1

Primary pollutants and other physico-chemical characteristics of soil samples Element / Parameter Pb Zn Fe Cu As Mn Cd S ClpH Specific Gravity

Value (mg⋅kg-1 dry soil) 49 705 69 486 235 905 5 873 5 760 14 994 100 52 320 5.0 8.1 4.38 (g⋅cm-3)

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CHINA PARTICUOLOGY Vol. 4, No. 2, 2006

Table 2

Bioavailable fraction of pollutants under investigation (leachable / ppm)

Pb 11 600

As 25 Table 3

Zn 956

Cu 577

Mn 253

Fe 1

Pollutants detected by XRD-analysis

Name of pollutant Lead (metallic) Lead Oxide Galena Anglesite Lead Carbonate Arsenopyrite Beudantite Adamite Iron (metallic) Hematite Wuestite Pyrite Troilite Zinc (metallic) Sphalerite Zinc Oxide Hauerite Valliminite

Chemical form Pb Pb3O4 PbS PbSO4 PbCO3 FeAsS PbFe(AsO4)(SO4)(OH)6 Zn2(AsO4)(OH) Fe Fe2O3 FeO FeS2 FeS Zn ZnS ZnO MnS (Cu, Fe)S2

Table 5

Mineralogical compositions of Greek FAs

Mineral Alumino-silicate Glass Quartz - SiO2 Anhydrite - CaSO4 Lime - CaO CaCO3 (Calcite) Feldspars - (Na, Ca)AlSi3O8 + Main phase; - Minor phase Table 6

Ptolemais FA + + + + + -

Megalopolis FA + + + + -

XRD identification of synthetic zeolites

Zeolite and zeolite-type material NaP1(Na6Al6Si10O32⋅12H2O) NaP (Na3.6Al3.6Si12.4 O32⋅12H2O) Herschelite (Na1.08Al2Si1.68 O7.44⋅1.8H2O) Tobermorite (Ca(OH)2Si6 O16⋅4H2O) Hydroxy-cancrinite (Na14Al12Si13O51⋅6H2O) Hydroxy-sodalite (Na1.08Al2Si.68O7.44⋅1.8H2O)

ZP50 +

ZP100

ZM50 +

ZM100

+

+

+

+ + + + +

2.3 Soil washing and pollutants mobilization Chemical analyses and mineralogical compositions of fly ashes (FA) obtained from the Greek lignite power plants of Ptolemais (PFA) and Megalopolis (MFA) are presented respectively in Tables 4 and 5. Both PFA and MFA were subjected to an alkaline hydrothermal treatment at 90°C, using NaOH 1M as an activation solution, in a 1 L stainless steel reactor, with an incubation period of 24 h and under mixing at 150 rpm and under atmospheric pressure. The mixture was then filtered using a 0.45 μm paper, and the filter cake was dried at 40°C for 24 h and then leached with water until no NaOH was detected. The final solid product was subjected to XRD analysis and SEM investigation for identification of known zeolites. The pH, CEC and SSA of the final solid zeolitic products (after drying and storage for several days) were also determined (Moutsatsou et al., 2006]. Two different solution/FA ratios were examined for each of PFA and MFA. Table 6 presents the XRD identification of the four synthesized materials. Table 4

Chemical analyses of the Greek FAs (% w/w)

Compound SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O Loss of Ignition

Ptolemais FA 30.16 14.93 5.10 34.99 2.69 6.28 1.01 0.40 3.95

Megalopolis FA 51.26 19.39 8.44 11.82 2.27 2.91 0.53 1.81 1.67

Soil and 1M HCl were mixed with a soil/liquid ratio of -1 30 g⋅L , and the suspension was stirred at 150 rpm, at room temperature, 20°C. Filtrates were collected at 1, 2, 4 and 8 h and analyzed for Pb, As, Zn, Cu, Cd, Fe and Mn by Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). All measurements were replicated twice, and the difference between the two measurements was limited to less than 5%. 100 90 80

Mobilization / %

2.2 Zeolite synthesis

70 60 50 40 30 20 10 0 0

Fig. 1

2

4

Time / h

6

8

Fe

Cu

Zn

Mn

Pb

As

Cd

%Overall

10

Pollutants mobilization as a function of time.

2.4 Pollutants retention Each of the above filtrates was mixed with each of the zeolitic materials prepared, in a soil/zeolite ratio of 3/1. The mixtures were agitated at 150 rpm for 2 h at room temperature, and the metal and metalloid contents were determined in the liquid by AAS and ICP-AES. Fig. 1 illustrates pollutant mobilization as a function of time.

Moutsatsou & Protonotarios: Remediation of Polluted Soils by Treated Fly Ashes

2.5 Pollutants Stabilization/Solidification (s/s) Pollutants stabilization/solidification was studied for three different cases: z Retention products were left to solidify on their own, trying to exploit the pozzolanic attributes of the fly ashes; z One part of lime (L) was added to the retention products; z One part of cement (C) was added to the retention products. For each of the above, compressive strength and setting time (Table 8), and the leaching behaviour (Table 9) of the stabilized /solidified products were determined.

3. Results and Discussion 3.1 Soil samples As shown in Table 1, the metal and metalloid concentrations in soil samples are high, much exceeding the international limits for industrial areas (European Environmental Agency, 1999). The age of contamination has rendered both the soil-metal and the metal-metal bounds less mobile due to the formation of numerous surface complexes of the solids. Another point of concern is that the metal sorption capacity of soil has well been exceeded, so that pollution is probably present as discrete metal-mineral phases instead of being bound to specific soil components (Kedziorek & Bourg, 1999; Moutsatsou et al., 2004). In Table 1 the soil pH shows that the soil is neutral, while the soil specific gravity is very high due to the presence of numerous metallurgical phases. The thermal behavior revealed a low organic content and a significant mass reduction only after 750°C, which is probably attributable to the decomposition of CaCO3 and sulfur compounds. The bioavailable fraction of pollutants, as shown in Table 2, was found to be extremely high in case of lead, while in case of zinc, arsenic, copper and cadmium, they, too, all exceed their respective international standards (European Environmental Agency, 1999). Mineralogical analysis of the soil sample (Table 3), demonstrated the presence of many different lead compounds, such as galena (PbS), anglesite (PbSO4), oxide, carbonate and complexed lead-iron and lead-arsenic compounds. Zinc appears mainly as sphalerite (ZnS) and secondarily as oxide and adamite. Copper is detected only in complexed ferric compounds, such as valliminite, while manganese is primarily present as hauerite (MnS), oxide and in lead-manganese compounds. The predominant form of iron is pyrite (FeS2), while many different iron oxides are also present. Finally, arsenic usually accompanies lead, iron and zinc in different mineralogical phases. These pollutant forms were expected due to the disposal of different types of wastes over the years and the continuous effect of air and rainfall.

3.2 Zeolite synthesis Table 6 illustrates the zeolites identified, along with their

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chemical forms. XRD pattern and SEM photographs of ZP50 and ZM50 (Moutsatsou et al., 2003; Moutsatsou et al., 2004; Moutsatsou et al., 2006) confirm an essential dissolution of the predominant fly ash phases and the formation of NaP1 (Na6Al6Si10O32⋅12H2O) and NaP Zeolite (Na3.6Al3.6 Si12.4O32⋅12H2O). Zeolitization yields fluctuate between 30-45%, depending on the substrate, with ZM50 presenting the best results (Moutsatsou et al., 2006). When the fly ash/NaOH ratio increases, less significant dissolution of fly ash is expected, as the latter is confirmed by XRD and SEM investigation of ZP100 and ZM100, since the activation of the predominant fly ash phases is low and both NaP and NaP1 appear in lower levels (Moutsatsou et al., 2006). ZP100 and ZM100 are thus expected to act more as a fly ash and less as a zeolite. The presence of non-reacted fly ash could be a critical parameter with respect to pollutants retention, since it may trigger different retention mechanisms, other than ion- exchange which is the predominant in case of zeolites. These mechanisms may include surface absorption and precipitation. On the other hand, fly ash may develop its pozolanic attributes for the effective stabilization/solidification of the final product.

3.3 Soil washing and pollutants mobilization Hydrochloric acid, has already been utilized by many researchers for purposes of extracting metals contained in soil (van Benschoten et al., 1997; Tampouris et al., 2001; Tokunaga & Toshikatsu, 2002). Literature results showed excellent performance for Pb and Zn, intermediate for Cu and rather low for As. In most studies, the yield of extraction increased with the density of HCl solution or by importing fresh HCl solution. Nevertheless, there are several factors that differentiate the case of the polluted soil coming from Lavrion (Moutsatsou et al., 2005): a) the majority of the researchers dealt with artificially polluted samples; b) metal formations in synthetic soil samples are usually simple, i.e., single metal compounds contrary to real mining-metallurgical waste (Davis & Singh, 1995), and therefore, the interaction between the complicated metal forms and the solvent, will probably differentiate metal mobilization rate; and c) most of the studies dealt with a restricted number of metals (maximum 3). Thus, 1 h of mixing is usually enough for pollutants extraction, independently of their positions in soil matrix and the selectivity of the solvent. Results presented in Fig. 1, support the aforementioned statements for the differentiation of Lavrion samples and render the kinetic studies crucial for the optimization of pollutants removal. Figure 1 illustrates the percentage of metals extracted from soil samples as a function of time (Moutsatsou et al., 2003; Moutsatsou et al., 2005). Hydrochloric acid performs satisfactory for all metals, especially for low and intermediate mixing times. In particular, the time for optimum pollutants removal can be spotted after 2 h of mixing (Moutsatsou et al., 2003; Moutsatsou et al., 2004).

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3.4 Pollutants retention Table 7 presents results of the retention experiments, showing that substrate ZP50 possesses satisfactory retention capacity for Pb, As, Fe and Mn under acidic environment, ZP100 demonstrates the highest yields in case of Zn and Fe, while ZM100 is very selective with respect to Cu. Finally, relatively low retention is achieved for Cd in all cases. Pure zeolites are expected to retain metals mainly through an ion-exchange mechanism. It should be mentioned that, for purposes of effective retention of metals and metalloids, the non-complete transformation of FA to zeolites is preferable. This is due to the fact that the portion of FA that has not been converted to zeolite, may trigger Table 7

additional retention mechanisms such as precipitation or surface adsorption. Another important advantage concerning the presence of unconverted FA is the capability of further stabilization/solidification of the pollutants on the substrates (Moutsatsou et al., 2004). Results presented in Table 7 are quite satisfactory, bearing in mind the extreme pollution load of the washing solution and the great variety of metals and metalloids present, along with the complicated mineralogy of real pollution. It is obvious that, in most cases, the comparable results between the different substrates verify the triggering of a variety of possible retention mechanisms, depending not only on the zeolitic material, but also on the pollutant-solvent-substrate interaction.

Pollutants retention on different zeolitic materials (%)

Fe

Cu

Zn

Mn

Pb

As

Cd

Total metal content / %

ZP50

28.7

50.0

18.5

23.1

40.7

32.9

13.3

28.3

ZP100 ZM50 ZM100

30.9 12.2 11.0

35.7 53.3 66.7

29.6 14.8 3.7

18.8 18.9 17.6

18.8 29.4 32.3

21.4 22.6 27.4

0.0 14.4 12.3

28.4 15.7 13.3

3.5 Pollutants stabilization/solidification Table 8 presents compressive strengths of all stabilized/solidified products. As expected, products with the addition of cement present higher strengths and the lower setting time, followed by those with the addition of lime. It should be mentioned that in case of disposal or land filling of stabilized/solidified waste when no cement is used as an s/s agent, there is no legislative restriction concerning waste compressive strength (Moutsatsou et al., 2004). Leaching behavior is ameliorated in the case of Fe (Table 9), when lime or cement is present. Copper presents very satisfactory behaviour for all the s/s products, while zinc is highly problematic for all zeolitic materials when additives are absent. Similar to zinc, Mn is leached in great percentages for ΖP50, ΖΜ50, ΖP100 and ΖΜ100. Lead lies within the respective standards for all the substrates. Arsenic Table 9

leaching behaviour is rather similar for all tested s/s products while Cd slightly exceeds the leaching limits in case of ZM50 and ZM100. Table 8

Compressive strength of stabilized/solidified products

Substrate ΖP50 ZP100 ZP50 + L ZP100 + L ZP50 + C ZP100 + C ΖΜ50 ΖΜ100 ΖΜ50 + L ZM100 + L ZM50 + C ZM100 + C

Compressive strength / kN 1.1 1.0 1.4 1.4 1.9 2.0 1.1 1.0 1.5 1.5 2.0 1.9

Leaching behaviour of pollutants after stabilization/solidification (mg⋅L-1 – DIN 38414/S4 )

Substrate ΖP50 ZP100 ZP50 + L ZP100 + L ZP50 + C ZP100 + C ΖΜ50 ΖΜ100 ΖΜ50 + L ZM100 + L ZM50 + C ZM100 + C Limits [EN 12457/1-4]

Fe

Cu

42.8 22.7 2.6 0.5 1.0 1.0 10.1 22.7 6.9 2.5 2.7 2.9 -

6.0 20.4 6.7 5.4 5.5 3.2 16.5 20.4 3.1 3.4 3.8 4.1 100

Zn 949.8 960.6 19.9 9.4 10.7 14.0 956.4 960.6 22.3 9.7 10.1 11.0 200

Mn 786.0 1012.0 1.0 1.0 1.0 1.0 946.2 1012.0 1.0 1.0 1.0 1.0 -

Pb

As

Cd

42.0 30.0 35.0 32.5 40.1 29.0 45.0 40.0 37.0 35.5 31.0 31.0 50

35.0 24.0 24.0 20.5 29.0 21.0 37.0 34.0 24.5 24.2 26.3 26.3 25

6.9 7.2 5

Setting time / h 168 168 96 96 24 24 168 168 96 96 24 24

Moutsatsou & Protonotarios: Remediation of Polluted Soils by Treated Fly Ashes

4. Conclusions The present paper deals with the application of a mobilization-retention-stabilization/solidification process for remediation of a highly metal-contaminated soil, containing slag and sulfur compounds waste. Hydrochloric acid has been proved effective for optimum pollutants removal at low and intermediate mixing times. Retention results showed that the yield of the process depends on a number of factors such as the metal speciation in soil and the interaction of solvent with both the soil and the substrates. These factors, along with the nature of the retention agent, differentiate the mechanisms by which pollutants are retained on substrates. Stabilization/Solidification results indicated the necessity of utilizing additional pozzolanic materials (such as lime and cement), aiming to upgrade the physical and mechanical properties of the retention agents and the leaching behavior of specific metals present in stabilized/solidified waste.

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