CHROMIUM SPECIATION IN MUNICIPAL SOLID WASTE: EFFECTS OF CLAY AMENDMENT AND COMPOSTING

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The addition of clay in the form of bauxite refining residue (red mud) prior to composting has been suggested as a way to control heavy metal mobility in compost. Leachability and plant availability of metals in a mixture of grass clippings and sawdust spiked with metal solution was markedly reduced during the composting process.

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Nội dung Text: CHROMIUM SPECIATION IN MUNICIPAL SOLID WASTE: EFFECTS OF CLAY AMENDMENT AND COMPOSTING

Pergamon Wat. Sci. Tech. Vol. 38, No. 2. pp. 17-23, 1998. IAWQ 0 1998 Published by Elsevier Science Ltd. hinted in Great Britain. All rights reserved 0273-1223/98 $19fKl+ OCG PII: SO273-1223(98)00427-2 CHROMIUM SPECIATION IN MUNICIPAL SOLID WASTE: EFFECTS OF CLAY AMENDMENT AND COMPOSTING Goen Ho and Liang Qiao Institute for Environmental Science, Murdoch University, Murdoch 6150. Australia Western ABSTRACT The addition of clay in the form of bauxite refining residue (red mud) prior to composting has been suggested as a way to control heavy metal mobility in compost. Leachability and plant availability of metals in a mixture of grass clippings and sawdust spiked with metal solution was markedly reduced during the composting process. The fate of metals in municipal solid waste compost applied to land was examined by using a sequential step extraction to investigate metal speciation (into exchangeable and bound to carbonate forms, to Mn & Fe oxides, to organic matter and in residue phase) in red mud amended compost. The effects of red mud and the composting process on metal speciation in the compost for Cd, Cr, Cu. Ni, Pb and Zn were investigated, and a comparison of some effects with biosolids compost was made. Addition of red mud reduced the metal mobility and the potential hazard of releasing metals from compost through promoted precipitation, adsorption and complexation of free metal cations to red mud. Red mud however, was not able to desorb metals bound to organic matter. Since most of the metals in the municipal solid waste were. not usually bound to organic matter, the addition of red mud prior to composting fixed the free metal ions before they bound to this fraction. Results for Cr speciation are reported in this paper. 0 1998 Published by Elsevier Science Ltd. All rights reserved KEYWORDS Bauxite refining residue(red mud); chromium;composting; metalmobility; metalspeciation;municipal solidwaste. INTRODUCTION Compost produced from mixed MSW can becontaminated heavymetals by derivedfrom contamination of domestic wasteby dry cell batteries, metalcoatings, paints,solvents, cosmetics,dyes,pesticides,lubricants and metals electronicequipment other discarded in and domesticappliances. way of overcomingthe One potentialheavy metalproblemof compost produced from mixed MSW is to add clay particles(< 2 pm) whichhavea largesurface areaandcapacityto adsorb heavymetals, reducing leachabilityandplant thus the availability of heavymetals thecompost. in Hofstede(1994) investigated immobilisation heavy metalsin the compost artificial MSW using the of of bauxiterefining residue mud)amendment. mixtureof grass (red A clippingsandsawdust spikedwith a was metalsolutionto makeartificial MSW, amended red mudandthencomposted controlledlaboratory with in 17
18 G. HO and L. QIAO incubators. The heavy metal leachability, plant availability and the total metal content in the artificial compost was determined by CaClz and DTPA extraction and acid digestion (HNO,, HCIO, and HCl) respectively. It was found that the addition of red mud prior to composting not only reduced leachability and plant availability of heavy metals in the compost, but also significantly reduced the levels of metals extractable by acid digestion. Red mud has a high pH buffer and cation exchange capacity (CEC), is high in Al and Fe oxides and can effectively adsorb free cations from solution. The metals in MSW compost that had been amended with red mud were found to be fixed through precipitation, adsorption and complexation with the latter. A reduction in metal mobility occurred when red mud was added prior to the composting of MSW, but only leachable metal levels were reduced when red mud was added to mature MSW compost (Hofstede and Ho, 1991). The metals in MSW became bound to the inorganic components in red mud before they came into contact with the organic fraction and were thus less available to plants. The mobility of metals in wastes or soils depends on metal concentration, speciation, pH, organic content and redox potential of the substrate. The high alkalinity, CEC and Fe and Al oxides content in red mud promote the fixing of the free metal ions through metal precipitation as hydroxides and carbonates, adsorption onto oxides and mineral surfaces and chemisorption or complexation with inorganic ligands. The ratio of soluble/exchangeable metal to total metal content is therefore an important factor in assessing the role of clay addition in controlling the mobility and plant availability of metals (Qiao et al., 1993). In order to determine the long-term fate and potential hazard of land application of MSW compost, it is necessary to investigate the speciation of heavy metals. Difficulties in studying this are exacerbated by the complex composition of MSW, which changes according to its source, location and season. It is thus very difficult to define a typical metal speciation in MSW. In general, the speciation of heavy metals in MSW tends toward more soluble and leachable metal species as a result of their sources not having an opportunity to mix and to complex with the organic matter. In the research reported in this paper, MSW samples were taken from Hofstede’s (1994) laboratory MSW composting experiment. The metal speciation in the compost, and the effect of both red mud and the composting process on metal speciation in these samples were investigated. Results for Cr are reported in this paper. MATERIALS AND METHODS Samples of MSW compost: MSW compost samples were from Hofstede’s (1994) batch experiment. There were seven incubators in a fully controlled laboratory composting system, which maintained the compost temperature at a target temperature of 55°C. This was accomplished by automatic adjustment of the aeration rate by a computer controlling convective and evaporative heat losses. Drying of the substrate was avoided by the passing of all inlet air through an air humidifier (Hofstede, 1994). The incubator containing the control sample had a mixture of 8 kg grass clippings and 2 kg sawdust based on wet weight (artificial MSW). The other incubators contained metal-spiked artificial MSW and dried red mud at 0 (red mud blank), I, 2, 3 and 4 kg. The blank MSW and the MSW with red mud addition were spiked with heavy metals at the following levels of dry MSW matter (salt used): lOmg/kg cadmium (Cd(NO&), 50 mg/kg chromium (CrCl@H,O), 50 mglkg lead (Pb(NO&), 20 mglkg nickel (Ni(NO&6H,O), 100 mg/kg copper (CuSO,*SH,O) and 500 mg/kg zinc, (ZnS04*7H,0). The mixtures were composted for 20 days, after which samples of MSW compost were dried at IOS’C and ground before metal extraction for determination of metal speciation. Drying a sample will change the metal speciation in the sample and make the metals more available (Qiao and Ho, 1997). The speciation in a moist sample will also change however, due to decomposition of organic matter during storage (Qiao et al., 1993). It was hence decided to store samples in a dry condition prior to extraction. Metal extraction: Approximately 1 gram samples (based on dry matter) were employed for the metal extraction. A sequential step extraction was carried out employing IM MgC12 (exchangeable fraction); 1M HOAc/NaOAc at pH 5 (carbonate); 0.04 M NH,OHHCI at 96°C (reducible or bound to oxides); 30% H~0~13.2 M NH40Ac at pH 2 and 85°C (bound to organic matter) extractions and concentrated nitric,
Chromium speciation in municipal solid waste 19 perchloric and hydrochloric acid digestion (residue fraction) (Tessier er al., 1979). Metals bound to sulphides in this extraction scheme would be included in the organic bound fraction. Two batch extractions were conducted by Hofstede (1994) employing 0.01 M CaC12 and 0.1 M DTPA followed by an acid digestion (HN03-HCl04-HCl) to estimate leachable, plant available, and total metal content respectively. Red mud neutralised with gypsum was also analysed to determine the speciation of the metal content in the mud. Samples and extractants were placed in closed centrifuge tubes shaken on a Coulter mixer for 12 hours, which was enough time to reach solution equilibrium, and the residues were separated by Sorvall RC- 5B ultra centrifuge at 10,000 rpm for 20 minutes. The supematant was passed through a GFK glass filter and stored at 4”C, before the residue was subjected to the next step of extraction. Six metals (Cd, Cr, Cu, Ni, Pb, Zn) were chosen for analysis, because they represented the heavy metals of interest in compost. The metals were analysed in duplicate on a GBC atomic absorption spectrometer. All reported metal figures in this paper were based on dry weight unless otherwise specified. Only the results for Cr are reported, because of limits to the length of this paper. Results for other metals will be reported elsewhere. RESULTS AND DISCUSSION Statistical analysis showed that both red mud addition and the cornposting process had a statistically significant effect on all measured metal concentrations at a < 0.05. Total metal concentration In ascertaining the concentration of total metal content in the compost (which could be used as a reference for metal distribution in the compost), two different kinds of independent measurement were carried out. These were the direct measurement and the sum of the metal in sequential extraction fractions. It was expected that the sum of the metal in sequential extraction would contain larger analytical measurement errors due to its multiple extractions and analysis whereas the direct measurement would give the more accurate result for total metal concentration. Because compost was spiked with known quantities of metals, the metal recovery rate by each measurement method was also calculated. The direct measurement gave 102% recovery rate (Hofstede, 1994), while the recovery rate calculated from the sum of the metals in sequential extraction fractions was 82% (Qiao, 1996). The total concentration of metals determined by direct measurement of dried and ground samples was shown in Table I. Although the total metal concentration in the blank samples should have been the sum of the metals in the control samples plus the amount spiked, this did not occur for all metals because the heavy metal solutions could not be completely homogenously mixed into the compost. However, the results of the total metal concentration were sufficiently accurate to assess the effect of red mud addition. The cornposting process concentrated the total metal in the compost due to loss of organic matter via decomposition, which was 24% on average (Hofstede, 1994). Table 1. Total metal content in MSW compost (mg/kg dry matter) After 20 days composting Initial concentration Spiked Metals& RM%+ Control 1 Blank 1 10% 1 20% 1 30% Control1 Blank 10% I 20% I 30% 73fo.6 1 &O.l I10&0.1 1.3f0.3 1 38f0.3 1 5tfO.6 1 73fl.7 1 74f0.1 4.2fO.21 ~$0.7 Cr 50 RM %I = percentage of red mud addition. Note: f number is standard deviation; The compost mixture with 10% of red mud addition in Hofstede’s (1994) experiment consisted of 8 kg grass clipping and 2 kg sawdust (the mixture had 60% moisture) and 1 kg red mud (dry). Based on dry matter weight, the 10% red mud addition was equivalent to l/(4+1) = 20% red mud, (similarly for the 20% (wet weight) is 2/6=33% (dry weight) and for 30% is 3fl = 43%). The metal concentration in the compost was calculated from the metal originally in MSW, metal added by spiking, metal in red mud and loss of compost weight through cornposting, and compared to the measured value (Table 2). The metal content of red mud is shown in Fig. 3.
20 G. HO and L. QL40 Table 2 Total metal content in compost (mg/kg); Comparison between measured and calculated values beginning At After 20 days composting I I RM%-B 1 1 10% 1 lo%*I 20% 1 209b’l 30% [309b*l 10% I10%*( 20% I 20%*( 30% 1309P 1 51 I 77 I 73 I 104 I 74 I 121 I 13.3 1 92 1 82 I 114 I 100 I 131 Cr 1 50 RM% = percentageof red mud addition; * = calculatedvalues(see text) Note: The net recovery rate of total metal content from the compost was 63% on average and 61% for Cr. These results showed that part of the metals in the compost was not recoverable by acid digestion. The metals in the spiking solution were present in ionic form and were adsorbed by red mud when the red mud was mixed with MSW. The metal ions, once adsorbed by red mud, were so strongly bound that they could not be recovered even under strongly acidic conditions (Hofstede, 1994). Leachabilitv of heavv metals Over 80% of the total metal content in the artificial MSW was derived from the spiking and therefore the mobility of metals could be expected to be higher in the MSW compost than in sewage sludge. For example, there was about 20% Zn in leachable form and 77% Zn in plant available form in the blank MSW sample, but in sewage sludge only less than 1.5% Zn was in leachable and 29% Zn in plant available form (Qiao and Ho, 1997). The higher metal mobility in MSW was also confirmed by Hofstede’s (1994) pilot plant composting experiment, in which actual domestic MSW was used to investigate the effect of red mud addition on metal leachability and plant availability. q 0 day of composting q 20th day of composting 82 Red Mud Addition (%) Figure I. The leachable metal content in municipal compost. Note: The error bar is the standard error of the data. Because of the high mobility of heavy metals in the compost mixture used, red mud had a marked effect on reducing the mobility of heavy metals in the compost. With only 20% of red mud addition the leachable metals were reduced by more than 90% in the MSW because the spiked metal ions readily reacted with red mud. The cornposting process also contributed to a decrease in the amount of leachable metals as a result of metal ions binding to the insoluble humin fraction that was produced in the composting process (Fig. 1). In contrast to the decrease of soluble organic matter during sludge composting, it gradually increased during the composting of MSW although the increment was diminished by the addition of red mud (Hofstede, 1994). This was due to adsorption of soluble organic matter to red mud. Plant availabilitv of heavy metals Reduction of the plant available metal levels in MSW by red mud (Fig. 2) was partially due to the leachable metal forming part of the plant available metal fraction. The composting process without red mud addition also decreased the plant available metal content. This change was probably a result of the redistribution of metal speciation during composting by humification of the organic matter.
Chromium speciation in municipal solid waste 21 0 0 day of cornpasting 6 q 20th day of composting Cr (w/kg) 4 2 0 Control Blank 10% 30% 20% Red Mud Addition (%) Figure 2. The plant available metal content in MSW compost. Note: The error bar is the standard error of the data. Metal sueciation The effect of red mud on the speciation of metals in compost is dependent upon its own metal speciation characteristics (Fig. 3). More than 60% of the metals were in residue form except for Zn which was distributed more evenly into the five fractions. This indicated that the metals contained in red mud were mainly in very stable forms even though the Cr content in the red mud was as high as 230 mg/kg. This fact was not surprising since red mud undergoes severe extraction processing (size reduction, Bayer process caustic digestion, and countercurrent washing) and has very little organic matter associated with it. 20.1 7.6 21.9 #230 33.3 # = Total metal OwYW q Exchangeable q Carbonates H Iron oxides bound q Organic fraction Residue No detectable Cd in red mud. Metals Figure 3. The speciation of metals in red mud The speciation of metals in the samples was markedly affected by the addition of red mud. Since different heavy metals had different properties and concentrations in the MSW compost, the speciation of metals and the effect of red mud were different for each metal. Chromium has an electron configuration closest to a noble gas with a high spherical symmetry and the lowest polarisability among the six tested metals. Furthermore, the Cr cation has a valency of three, and therefore has a stronger electrostatic affinity for the sorption sites than the other divalent metal cations. Cr thus formed the most stable complexes among the six metals and dominated in the residue and organic bound fractions. Because Cr prefers to form stable complexes with ligands and to be adsorbed on surface sites, the spiking metal solution not only increased the exchangeable Cr (as with the other metals), but also the Cr bound MSW more than was observed increased to organic and oxides in the blank with other metals. The Cr speciation in the control samples may have had large analytical errors due to the low Cr content (cl.5 mg/kg).
22 G. HO and L. QIAO The cornposting process did affect the speciation of Cr in the compost though the magnitude of changes was not marked. The carbonates and oxides bound Cr were converted into the organic bound fraction during the cornposting process as the result of the competition of Cr with other metal cations for the limited humic organic ligands produced during MSW cornposting (Fig. 4). This increase was slowed down by red mud addition. The effect of the cornposting process on Cr speciation was similar to that with biosolids cornposting (Qiao, 1996). This conversion increased the stability of Cr complexes in the mature biosolids compost. Like the Cr in biosoiids (Qiao and Ho 1997), the speciation of Cr in the MSW compost was changed by red mud amendment due to the high Cr concentration in the red mud (Fig. 4). After factoring out the red mud dilution effect, the residual Cr appeared to have a negative value due to the lower recovery rate (61%) of total Cr from the compost. About 40% of the total Cr was converted into irreversible forms which could not be extracted even by strong acids. From this, it was surmised that although some Cr may have become available with changed environmental conditions, much of it was unlikely to be released. 1.4’ 39 49 52 64 1.5 58 66 74 68 100 ‘-total Cr (mgikg) 60 aQ q exchangeable 60 t q carbonates q 40 iron oxides bound III organic fraction n residual 20 0 (a) --- At beginning of cornposting (b) --- After composting for 20 days Control --- no red mud and no spiking with metal solution Blank --- no red mud, but spiked with metal solution Figure 4. Distribution of Cr in MSW compost. correlation between leachable. plant available metals and metal sveciation The relationship between plant available metal and the metal in exchangeable, carbonate and bound-to- oxides fractions is very close. It can be seen that DTPA can generally extract the heavy metals in the soluble, exchangeable and carbonate bound fractions except for Pb (Fig. 5). Petruzzelli (1989) stated that the metals present in these fractions are considered to be the most available forms for plants. Since the reagent MgCI, for the extraction of exchangeable metal (I M) has a higher concentration than the reagent CaCI, used for the extraction of leachable metal (0.01 M), the exchangeable metal concentration is higher than the leachable metal concentration. The metals in the exchangeable form may leach from the waste under certain condition such as acidic rain, or saline wastewater irrigation.
Chromium speciation in municipal solid waste 23 cd Cr Cu’ NI al Metal Metal Figure 5. Comparison of metal extracted by DTPA with the metals in soluble and weakly bound fractions in the MSW compost. Note: Sum = Sum of exchangeable and carbonate fractions; * = Sum of exchangeable, carbonate and bound to oxides fractions. CONCLUSIONS Metal in the MSW tend to be more mobile than in biosolids and red mud addition more strongly changes the metal speciation in the MSW compost. Composting process reduces the mobility and plant availability of metals in the MSW even though the total concentration of metals increased due to reduction in compost weight during the composting. Red mud reduces by about one third the total metal content in the MSW compost that can be extracted by strong acid digestion (HNO3 + HCl04 + HCl). Red mud addition significantly increased the total Cr content in the red mud MSW as the result of high Cr contained in the red mud (230 mg/kg). In spite of increasing of the total Cr content, the mobility and plant availability of Cr was still very low. Factoring out the dilution effect of red mud, there is about one third of Cr unextractable by the acid digestion. Red mud addition has a stronger ability to immobilise the heavy metals in MSW than it does in biosolids. The key point of metal immobilisation through the red mud addition is the fixing of the free metal ions before it forms complexes with the organic matter in wastes. Therefore. it is more effective if the red mud is mixed with the wastes prior to the composting process. The reason is the red mud cannot desorb the organic bound metals to form the inorganic complexes with these metals. Although it cannot desorb the organic bound metals, red mud addition can still effectively limit the potential release of the organic bound metals when the organics were decomposed. REFERENCES H. (1994). Use o f bauxite refining residue to reduce the mobility of heavy mefals in municipal Hofstede. solid wa.ste compost. Ph D thesis, Environmental Science, Murdoch University, WA 6150, Australia. Hofstede. H. T. and Ho, G. E. (1991). The effect of the addition of bauxite refining residue (red mud) on the behaviour of heavy metals in compost, In: Trace meralr in the Environment, Vol. I: Heuuy MeraLr in the Environment, J.- P. Vemet (ed). Elsevier, Amsterdam, pp. 67-94. Petruzzelli. G. (1989). Recycling wastes in agriculture: heavy metal bioavailability. Agriculture, Ecosystems and Environment, 27, 493-503. Qiao, L., Hofstede, H. and Ho, G. E. (1993). The mobility of heavy metals in clay amended sewage sludge and municipal solid waste compost. In: Proceeding Toronto, Canada, 2, of 9th International Conference on Heavy Metals in the Environment, 450-453. Qiao, L. (1996). The mobility Ph D thesis, of heavy metals in clay amended sewage sludge and municipal solid waste compost. Environmental Science, Murdoch University, WA 6150. Australia. Qiao, L. and Ho. G. E. (1997). The effects of clay amendment and composting on metal speciation in digested sludge. Warer Research, 31.951-964. Tessier, A., Campbell, P. G. C. and Bisson, M. (1979). Sequential extraction procedun for the speciation of particulate trace metals, Analytical 51, 844-850. Chemistry,