Speciation and Removal of Zinc from Composted Municipal Solid

This paper presents composting of the organic fraction of municipal solid waste containing 50000 mg/kg of cellucotton and 7980 mg/kg of zinc carried o...
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Environ. Sci. Technol. 2001, 35, 810-814

Speciation and Removal of Zinc from Composted Municipal Solid Wastes T . K O R O L E W I C Z , † M . T U R E K , * ,† J. CIBA,† AND J. CEBULA‡ Institute of Chemistry, Inorganic Technology and Electrochemistry and Institute of Water and Wastewater Engineering, Silesian University of Technology, 44-101 Gliwice, Poland

This paper presents composting of the organic fraction of municipal solid waste containing 50000 mg/kg of cellucotton and 7980 mg/kg of zinc carried out under laboratory conditions. In the initial material as well as the compost obtained, zinc, cadmium, copper, nickel, and lead were analyzed, and their forms were determined by means of sequential extraction. It was found that 65% of zinc occurs in the organically bound form. Removal of zinc from the waste through leaching and subsequent electrochemical separation from the leaching solution was also examined. A double extraction of the waste with sodium diphosphate(V) enables a reduction of zinc content to 1240 mg/kg. As a result of electrolysis of the leaching solution, 90.2% of Zn is separated on the cathode. This paper suggests a method for processing municipal solid waste with high zinc content based on extraction of the waste with sodium diphosphate(V) and composting. The leaching solution is recovered electrochemically.

Introduction In Poland, industry and households produce 200 million tons of waste every year (which amounts to 40% of total raw materials, power, goods and products processed) (1, 2). Municipal solid wastes are disposed of in dumps, whose total area is 1.8-2.0‚103 m2, and their effect on the environment through dust pollution and contamination of groundwater with leachates causes degradation of areas much larger than those of the dumps they are stored in ref 1. Composting of municipal solid wastes is regarded as the most rational method as far as ecology is concerned (3). It is possible to produce high-value compost from organic wastes after the removal of ballast substances such as glass, ceramics, metals, plastics, and toxic substances, including heavy metals. Heavy metals are hazardous to ecosystems due to their adverse effect on the complex system of soil processes. An analysis of the researches on utilization of the wastes characteristic of high heavy metal content may lead to two main research trends: the first one deals with changing the metals into forms nonavailable to plants and animals and the other deals with removal of the metals to the required level (1, 3). * Corresponding author phone: +48 32 2372735; fax: +48 32 2372277; e-mail: [email protected]. † Institute of Chemistry, Inorganic Technology and Electrochemistry. ‡ Institute of Water and Wastewater Engineering. 810

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Generally, the removal of heavy metals from solid waste and soil is examined through leaching and electrokinetic method. Organic and inorganic acids, bases, complex compounds, and surface active agents can be used for the extraction removal of heavy metals from soil, including zinc and its compounds, using the off-site method. Extraction of heavy metals by inorganic acids and complexing agents has severe drawbacks. Organic acids could be an attractive extracting agent because the extraction can be performed under mildly acidic conditions (pH 3-5) and they are biologically degradable (4). A model was developed that allowed for the evaluation of a soil metal cleaning technique in a rapid and cost-effective manner. Metal flow (Pb, Cu, Zn, Cd) during a counter-current soil-acid extraction procedure, consisting of a decarbonation, solubilization, and washing step, was determined (5). Acid extraction of soils and metal removal from post-extraction solutions are described in refs 6 and 7. Laboratory studies were conducted on both acids and chelating agents to evaluate their ability in extracting heavy metals from contaminated soils (8). Barona et al. (9) evaluated the effects of chemical associations of zinc and copper on extraction with DTPA in soils. Peters (10) found that EDTA and citric acid appear to offer the greatest potential as chelating agents to use in soil washing. The overall removal of copper, lead, and zinc in his research from the multiplestage washing were 98.9%, 98.9%, and 97.2%, respectively. Oxalate (Ox) and EDTA were investigated as extractants for decontaminating Zn and Pb polluted soils. When contaminating metals are associated with soil oxides, Ox may be a superior extractant to powerful chelants such as EDTA (11). Hong et al. (12) tested the extraction, recovery, and biostability of EDTA as a potential remediating agent. They found that EDTA is a strong, recoverable, and relatively biostable chelating agent that has potential for soil remediation application. Electrokinetic remediation may be carried out using “ex situ” and “in situ” methods. Zagury et al. (13) studied at pilot scale the ex situ remediation of heavy metals contaminated industrial sludge. In the paper (14), EDTA-enhanced electroremediation of metal-contaminated soils was investigated. Lead and zinc were selected as contaminants. It was found that EDTA added to the catholyte can be readily delivered into a sandy soil where it solubilizes the precipitated metals. The resulting complexes are then transported to the anode with metal removal efficiencies, approaching 100%. Our previous research showed that cadmium, lead, and zinc present a problem in the municipal solid wastes from the agglomerated region of Silesia. Their amounts often exceed the standards for composts produced from the municipal solid wastes (15, 16). In the analyzed samples of the municipal solid wastes from Gliwice, the amount of zinc exceeds the permissible standards for the III class compost; lead and cadmium are within the standard for the II class; and the values of the other metals match the I class. In a composting plant in Katowice, the amounts of metals in the compost produced occur within a wide range: lead very often exceeds the value of 1000 mg/kg and zinc 3000 mg/kg. Thus, the compost becomes unsuitable for use (17). The application of acid leaching and electrochemical separation of metals from a leaching solution enables a separation of cadmium and lead in the amounts exceeding 80% of the initial content (16), thus obtaining a product which meets the requirements for the I class of the above-mentioned standard (15). Zinc, however, was extracted to a much smaller extent: its amount was reduced from 3400 mg/kg to 2490 mg/kg. The product obtained met merely the requirements 10.1021/es001441l CCC: $20.00

 2001 American Chemical Society Published on Web 01/18/2001

TABLE 1. Instrumental Parameters

TABLE 2. Total Content of Metals in the Modified Municipal Solid Wastes

element parameter

Cd

Cu

Ni

Pb

Zn

wavelength, nm band-pass, nm lamp intensity, mA air-acetylene flow rate, dm3/min

228.8 0.7 6 11-1

324.8 0.7 30 11-1

232.0 0.2 30 11-1

283.3 0.7 15 11-1

213.9 0.7 15 11-1

for the II class (15) (up to 2500 mg/kg), whereas the objective was to obtain a product suitable for the I class (up to 1500 mg/kg), which enables its application without major limitations. In Benelux, the amounts of zinc in municipal solid wastes range from 8000 to 10000 mg/kg. Largely, this is due to baby care products containing zinc oxides, which are disposed of with diapers (18). A similar situation may be expected in Poland in a few years’ time. This research is aimed at the following: observing changes in forms of zinc oxide during the composting process, evaluating the effect of this element on the degradation of organic substances, reducing zinc content by leaching wastes, and examining the possibilities of electrochemical separation of zinc from the solution after leaching. The research was conducted on a fraction of municipal solid wastes with increased content of cellucotton and zinc.

element

requirements of standards for compost, mg/kg (15)

total content of metals,a mg/kg

cadmium copper nickel lead zinc

5 300 100 350 1500

8.1 (0.051) 89 (0.064) 28 (0.0620) 475 (0.0286) 8050 (0.0247)

a Mean value of four samples. The relative standard deviation is given in brackets.

Experimental Section Apparatus. A Perkin-Elmer double beam Atomic Absorption Spectrometer model 3300 was used for the analysis of extracts by FAAS. Hollow cathode lamps of Cd, Cu, Ni, Pb, and Zn were used as radiation sources. The instrumental operating conditions are shown in Table 1. A multiposition shaker WU4, with continuously variable rotation control, was used in investigations of extraction. A WE 1 (5000 rpm) centrifuge was used for separation of the solid phase from the extractant liquid. Reagents and Materials. All chemicals used were of analytical-reagent grade. All solutions were prepared in deionized water. Stock solutions of analytes (1 g/dm3) were prepared by dissolving the pure metal or the appropriate salts and making up to volume with deionized water. Calibration standards were prepared by appropriate dilution of the stock solution with nitric acid (concentration of 0.1 mol/dm3) prior to use. An organic fraction of the municipal solid wastes from Gliwice (high buildings) constituted the initial material for the research. To reach the levels of zinc and cellucotton characteristic of Benelux countries (18) (i.e. approximately 8000 mg of Zn and 50000 mg of cellucotton per kg of dry solid wastes), the composition of the wastes was modified by adding zinc oxide and cellucotton. In the second test, the composition of the wastes was modified by adding (to the initial material) 50000 mg/kg of cellucotton only. Municipal Solid Wastes and Compost Analysis. The modified method presented by M. Pinta (19) was applied to determine the total zinc content in the municipal solid wastes with an addition of zinc oxide and in the composts with and without an addition of zinc oxide. 1.000-g samples dried at 105 °C were heated in a platinum crucible for 5 h at the temperature of 480 °C, cooled, and wetted with 2 cm3 of nitric acid (concentration of 1 mol/dm3), dried on a heating plate, and heated for 2 h at 450 °C. Then they were treated twice with 1 cm3 of 36% hydrochloric acid and 5 cm3 of 40% hydrofluoric acid (the content was evaporated to dryness each time). The residue obtained was subjected to hot digestion with nitric acid (1 mol/dm3), and the solution (after separation and washing of the sediment) was transferred into 25 cm3 volumetric flasks. The total content of metals in

FIGURE 1. Temperature during the composting process. the municipal solid wastes modified by adding zinc oxide and cellucotton, assayed by the FAAS method, is presented in Table 2. The table also shows the requirements concerning I class compost produced from the municipal solid wastes (15). Composting of Municipal Solid Wastes. Composting was carried out under aerobic conditions. 500 g of the solid wastes was ground and sieved through a 3-mm sieve, and then 200 cm3 of inoculum (water solution) and distilled water were added until the humidity of 50% was achieved. The mixture was then mixed. The substrate obtained was introduced into a 2.0 dm3 thermally insulated glass flask and aerated by pumping in 4 cm3/s of air through a sieve in the bottom of the flask. The process was observed by monitoring temperature changes. Comparative tests on composting unmodified wastes were also carried out. The results are presented in Figure 1. In the case of the wastes with the addition of zinc and cellucotton, the maximum temperature reached a mere 42 °C and decreased fast to ambient temperature. It may be assumed that the worse effects of the composting process of the modified wastes result from the increased content of zinc, because similar results were obtained in other investigations (20). Therefore, zinc should be removed from the wastes prior to composting. Metals Forms in the Compost. Sequential extraction constitutes the most common method for determining forms of metals in composts. It enables us to specify a relative distribution of particular forms of an element in its total content in wastes and consists of treating a sample successively with chemical solutions characteristic of different leaching power. In this research, the sequential extraction presented by Rudd (21) has been used. The extractants were as follows: water, potassium nitrate(V), potassium fluoride, sodium diphosphate(V), ethylenediaminetetraacetic acid sodium salt, and nitric(V) acid solutions. The forms determined in the solutions obtained are as follows: soluble, exchangeable, adsorbed, organically bound, carbonate, sulfide, and nonavailable. Extraction conditions are shown VOL. 35, NO. 4, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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a

Mean value of four samples. The relative standard deviation is given in brackets. b Signifies below the limit of detection of the given element under the determination conditions.

350 (0.102) 50 (0.260) 17 (0.196) 1626 1680 (0.053) 252 (0.015) 41 (0.145) b 494 500 (0.032) 9 (0.150) 4.5 (0.263) b 29 30 (0.048) 24 (0.047) 35 (0.038) 4 (0.206) 94 94 (0.027) 2.8 (0.084) 1.4 (0.138) b 8.0 8.5 (0.046) 330 (0.0889) 47 (0.123) 15 (0.173) 1554 1600 (0.035) 1100 (0.0992) 400 (0.0884) 130 (0.110) 7930 8050 (0.0413) 860 (0.0738) 300 (0.124) 100 (0.107) 7880 7980 (0.0398) EDTA HNO3 HNO3

H2O KNO3 KF Na4P2O7

A - soluble B - exchangeable C - adsorbed D - organically bound E - carbonate F - sulfide G - insoluble sum of form total content

0.1 6.0 14.0

60 (0.321) 30 (0.265) 20 (0.183) 1100 (0.094)

b 20 (0.310) 31 (0.234) 150 (0.087) b 5.5 (0.252) 6 (0.213) 4 (0.250) 3 (0.222) 5 (0.104) 13 (0.087) 10 (0.092)

b 1.1 (0.211) 1.1 (0.196) 1.6 (0.123) 56 (0.128) 28 (0.166) 18 (0.156) 1060 (0.0653) 240 (0.108) 160 (0.102) 100 (0.133) 5800 (0.0760) 750 (0.0940) 500 (0.0871) 170 (0.127) 5200 (0.102)

extractant

b 1.0 0.5 0.1

Zn Pb Ni Cu wastes not composted + cellucotton + ZnO

composted wastes + cellucotton + ZnO

composted wastes + cellucotton

Cd

determined contents of metal forms in composted wastes,a mg/kg determined content of zinc forms,a mg/kg

concn, mol/ dm3 form

where h - potential of hydrogen evolution, V; a - coefficient of hydrogen evolution over potential at current density of 1 A/cm2; b - coefficient; and i - current density, A/cm2. Coefficients “a” for lead and zinc were 1.53 and 1.24 V, respectively, and coefficients “b” were 0.12 V for both lead and zinc. The calculations results are shown in Table 5. Each time, 120 cm3 of the solution obtained from a single extraction of wastes was subjected to electrolysis. Taking into consideration the amount of zinc in the solution (Table

extraction conditions

h ) a - blni

TABLE 3. Content of Metals Forms Determined by Sequential Extraction

in Table 3. The following procedure has been applied: a homogenized waste sample treated with an extractant (solid phase: fluid phase ) 1:10 (m/m)) was shaken periodically (15 min/h) during 72 h and centrifuged. The solid phase, after being washed with water (solid phase: water ) 1:5), was treated with another extractant. The extract, after being diluted appropriately, was analyzed by the FAAS method. Extraction conditions and distribution of particular metal forms are presented in Table 3. Preparatory Metals Extraction. On the basis of the results obtained, it has been found that most metals present in the wastes may be extracted applying sodium diphosphate(V) and EDTA sodium salt. Therefore, they were used during the preparatory extraction of metals from the wastes. The wastes were divided into four 12 g samples which were placed in Erlenmeyer flasks and filled with 120 cm3 of sodium diphosphate(V) (concentration 0.1 mol/dm3) or 120 cm3 of EDTA sodium salt (concentration 0.1 mol/dm3). Then they were shaken periodically (15 min/h) during 72 h in a shaker. The solid phases were centrifuged, and the solutions obtained after filtration were analyzed and subjected to electrolytic separation of zinc. In the case of multiple leaching, 120 cm3 of Na4P2O7 or EDTA was added twice to the solid phase after the first leaching, with centrifuging the solid-phase each time. The solutions were analyzed to determine metals content by means of the FAAS. The results of preparatory zinc extraction are shown in Table 4. The content of the metals in the waste after double extraction with sodium diphosphate(V) are as follows (mg/kg): Cd - 4.3, Cu - 65, Ni - 15, Pb - 280, Zn - 1240. Thus the waste meet the requirements of the first class compost (see Table 2). Electrochemical Zinc Separation. In the post-extraction solutions, the concentration of zinc is much higher than those of the other metals. Thus, further tests focused on the possibility of recovery of the leaching solutions through zinc separation. Based on both our own previous investigations (16) as well as literature data (22) it was assumed that effective results of electrochemical separation of zinc from the solutions after wastes extraction were to be expected. The electrochemical separation of zinc could enable a reuse of the extractants. The solutions which resulted from the wastes extraction contain zinc in the form of a complex with P2O74- or EDTA with an excessive extracting agent. The concentration of zinc cation is therefore low, its value results from instability of certain complexes. High efficiency may be achieved by conducting electrochemical separation at the potential lower than hydrogen evolution potential. Zinc was separated on a lead electrode. The electrolyzer applied was equipped with an 18 cm2 cathode and two Ti/Pt anodes. Current density was 0.5 mA/cm2. Prior to electrolysis, the expected values of zinc and hydrogen discharge potential on the lead and zinc electrodes were calculated (in the initial stage of the process the cathode is coated with zinc and therefore operates as a zinc electrode). Potential of hydrogen evolution was calculated according to the Tafel equation

TABLE 4. Results of Preparatory Extraction extractant Na4P2O7 EDTA

extraction no.

amount of leached zinc,a mg/kg

extent of leached zinc, %

1 2 3 1 2 3

5300 (0.136) 1440 (0.180) 290 (0.223) 4520 (0.182) 1180 (0.243) 60 (0.304)

66.4 18.0 3.6 56.6 14.7 0.7

a Mean value of four samples. The relative standard deviation is given in brackets.

TABLE 5. Conditions of Electrochemical Zinc Separation extractant indicator

Na4P2O7

EDTA

pH constant of Zn complex stability potential of H2 evolution on Pb, V potential of H2 evolution on Zn, V potential of Zn discharge, V potential of Zn discharge after having separated 90% of its initial content, V

8.5 3.16 × 106 -1.628 -1.338 -0.942 -0.978

6.5 3.16 × 1016 -1.512 -1.222 -1.267 -1.298

TABLE 6. Electrochemical Separation of Zinc from Leaching Solution extractant zinc content,a mg/kg

Na4P2O7

EDTA

solution before electrolysis solution after electrolysis electrodes

5300 (0.138) 520 (0.128) 4780 (0.188)

4520 (0.100) 4200 (0.123) 320 (0.226)

a Mean value of four samples. The relative standard deviation is given in brackets.

4), it was estimated that the electrolysis should last for 6 h in order to achieve a complete separation of zinc. Due to the possibility of low current efficiency, especially in the case of EDTA as an extractant, the electrolysis lasted for 24 h. After the electrolysis, the electrodes were treated with a 3 mol/ dm3 hydrochloric acid solution, the solution obtained was evaporated to dryness, and the dry residue, after being solubilized in a 0.1 mol/dm3 nitric acid solution, was analyzed. The analysis covered also the electrolyte solution. The results of the electrochemical separation of zinc are presented in Table 6.

Discussion In the case of the unmodified wastes, there was a rapid increase in temperature during the first 24 h, which later decreased and stabilized at 30 °C, the ambient temperature being 22 °C. The obtained temperature over 50 °C (the maximum temperature was 59.5 °C) indicates that the process was carried out correctly. Composting the waste modified with only the addition of cellucotton resulted in similar temperature values, the maximum temperature being slightly lower and amounted to 55 °C. In the case of the wastes with the addition of zinc and cellucotton, the maximum temperature reached a mere 42 °C and decreased fast to ambient temperature. Since higher temperatures bring some positive properties to compost, and it did not happen in this case, the wastes did not have the properties characteristic of suitable compost caused by an excessive amount of zinc. Maintaining the composting temperature over 55 °C is advantageous because of the compost sanitation (pathogens cease to exist) (23).

FIGURE 2. Suggested scheme of zinc separation from the wastes. Sequential extraction (Table 3) proved that despite the unsuccessful composting of the modified wastes, there were some changes in occurrence of particular zinc forms: the soluble and exchangeable forms decreased considerably, while the organically bound (extracted with diphosphate(V)) and carbonate (extracted with EDTA sodium salt) forms increased. The fact that mixing zinc oxide with the wastes makes zinc change its form, mainly into an organically bound one, confirms the results obtained earlier (18, 20). The preparatory extraction proved that sodium diphosphate(V) extracts zinc more efficiently than EDTA sodium salt. A single extraction leaches 66.4% of zinc and a double one 88.4% (total). EDTA sodium salt achieved 56.6% and 71.3% respectively. Further leaching produces very poor results (3.6% and 0.7% respectively). A double extraction with sodium diphosphate(V) reduced zinc content in the wastes from 7980 to 1240 mg/kg, which corresponds to the first class compost (below 1500 mg/kg). At the same time, the contents of cadmium and lead are decreasing also to the level matching the I class compost. Electrochemical separation of zinc from leaching solutions was carried out under conditions presented in Table 5. As a result of electrolysis of sodium diphosphate(V) after zinc extraction, 90.2% of zinc present in the solution was separated on the cathode, whereas in the case of EDTA sodium salt only 7.1%. This met our expectations since in the case of EDTA sodium salt the potential of zinc discharge is higher than the potential of hydrogen evolution. The application of Na4P2O7 rather than EDTA is also supported by the fact that, as found (24), electrolysis brings about EDTA degradation, presumably due to its decarboxylation, which significantly reduces the possibility of reuse for leaching. The fact that EDTA is practically not biodegradable might be considered as its drawback (12). Based on the research results, a method for separation of zinc from the municipal solid wastes has been suggested (Figure 2). It consists of a double extraction of zinc with sodium diphosphate(V), which circulates in a closed cycle. The solution after the first extraction is subjected to electrolysis. The product obtained (wastes for composting) contains less than 1500 mg/kg of zinc. The content of the other metals after extraction is also small (section: preparatory zinc extraction). Thus the compost produced from it will meet the requirements of the first class compost. VOL. 35, NO. 4, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Literature Cited (1) Borkiewicz, J. Industrial waste management and ecology; Foundation for ecology “Silesia”: Katowice, 1993 (in Polish). (2) Brochure of Institute of Wastes Management in Katowice; 1994 (in Polish). (3) Ciurla, Z. Arch. Environ. Prot. 1986, 1-4, 185. (4) Veeken, A. H. M.; Hamelers, H. Water Sci. Technol. 1999, 40, 129. (5) Masscheleyn, P. H.; Tack, F. M.; Verloo, M. G. Water, Air, Soil Pollut. 1999, 113, 63. (6) Hasse, B.; Render, M.; Luhede, J. Chem. Ing. Tech. 1991, 63, 392. (7) Tuin, B. J. W. Dissertation, Technische Universiteit Eindhoven, 1989. (8) Neale, C. N.; Bricka, R. M.; Chao, A. C. Environ. Prog. 1997, 16, 274. (9) Barona, A.; Aranguiz, I.; Elias, A. J. Chem. Technol. Biotechnol. 1999, 74, 700. (10) Peters, R. W. J. Hazard. Mater. 1999, 66, 151. (11) Elliot, H. A.; Shastri, N. L. Water, Air, Soil Pollut. 1999, 110, 335. (12) Hong, P. K. A.; Li, C.; Banerji, S. K.; Regmi, T. J. Soil Contam. 1999, 8, 81. (13) Zagury, G. J.; Dartiguenave, Y.; Setier, J. C. J. Environ. Eng.-Asce 1999, 125, 972. (14) Wong, J. S. H.; Hicks, R. E.; Probstein, R. F. J. Hazard. Mater. 1997, 55, 61.

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(15) Branch Standard 90-9103-13, Composts from municipal wastes (in Polish). (16) Ciba, J.; Korolewicz, T.; Turek, M. Water, Air, Soil Pollut. 1999, 111, 159. (17) Schabowska, E. Composting plant in Katowice; personal communication, 1995. (18) Haug, R. T. The practical handbook of compost engineering; Lewis Publishers: Boca Raton, 1993. (19) Pinta, M. Spectrometrie d′absorption atomique applications a l′analyse chimique; Masson et Cie, Eds.; Paris, 1971. (20) Ciba, J.; Zołotajkin, M.; Cebula, J. Water, Air, Soil Pollut. 1997, 93, 167. (21) Rudd, T.; Lake, D. J.; Mehrotra, J.; Sterrit, R. M.; Kirk, P. W. W.; Campbel, J. A.; Lester, J. N. Sci. Total Envir. 1988, 74, 149. (22) Tho¨ming, J.; Colmano, W. Wasser Boden 1995, 47, 8. (23) Jokela, J.; Rintala, J.; Oikai, A.; Reinikainen, O.; Mutka, K.; Nyro¨nen, T. International Conference on Sludge Management, Cze¸ stochowa, 26-28 June 1997; Vol. 1, p 132. (24) Korolewicz, T.; Turek, M.; Ciba, J. Arch. Environ. Prot. 2000, 26, 83.

Received for review June 30, 2000. Revised manuscript received November 17, 2000. Accepted November 22, 2000. ES001441L