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Chromium speciation in municipal solid waste: Effects of clay amendment and composting
~
Pergamon
War. Sci. Tech. Vol. 38. No.2, pp. 17-23, 1998. IAWQ C 1998 Published by Elsevier Science Ltd. Printed in Great Britain. All rightJ reserved
PH: S0273-1223(98)00427-2
0273-1223/98 $19'00 + 0-00
CHROMIUM SPECIATION IN MUNICIPAL SOLID WASTE: EFFECTS OF CLAY AMENDMENT AND COMPOSTING Goen Ho and Liang Qiao Institute/or Environmental Science. Murdoch University, Murdoch 6150. Western Australia
ABSTRACf 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 rnaner, 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. ~ 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Bauxite refining residue (red mud); chromium; composting; metal mobility; metal speciation; municipal solid waste. INTRODUCTION Compost produced from mixed MSW can be contaminated by heavy metals derived from contamination of domestic waste by dry cell batteries, metal coatings, paints, solvents, cosmetics, dyes, pesticides, lubricants and metals in electronic equipment and other discarded domestic appliances. One way of overcoming the potential heavy metal problem of compost produced from mixed MSW is to add clay particles « 2 urn) which have a large surface area and capacity to adsorb heavy metals, thus reducing the leachability and plant availability of heavy metals in the compost. Hofstede (1994) investigated the immobilisation of heavy metals in the compost of artificial MSW using bauxite refining residue (red mud) amendment. A mixture of grass clippings and sawdust was spiked with a metal solution to make artificial MSW, amended with red mud and then composted in controlled laboratory 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 CaCI2 and DTPA extraction and acid digestion (HN0 3, HCI0 4 and HCI) 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 AI 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 AI 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): lOmglkg cadmium (Cd(N0 3h ), 50 mglkg chromium (CrCI306H20), 50 mglkg lead (Pb(N0 3h), 20 mglkg nickel (Ni(N03ho6H20), 100 mglkg copper (CuS0405H20) and 500 mglkg zinc, (ZnS0407H20). The mixtures were composted for 20 days, after which samples of MSW compost were dried at lO5°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 I gram samples (based on dry matter) were employed for the metal extraction. A sequential step extract ion was carried out employing 1M MgCI2 (exchangeable fraction); 1M HOAclNaOAc at pH 5 (carbonate); 0.04 M NH20HHCI at 96°C (reducible or bound to oxides); 30% H202/3.2 M NH40Ac at pH 2 and 85'C (bound to organic matter) extractions and concentrated nitric,
Chromiumspeciationin municipal solid waste
19
perchloric and hydrochloric acid digestion (residue fraction) (Tessier et 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 CaCI2 and 0.1 M DTPA followed by an acid digestion (HN0 3-HCI04-HCI) 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 RC5B ultra centrifuge at 10,000 rpm for 20 minutes. The supernatant was passed through a GF/C 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 composting 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 composting process concentrated the total metal in the compost due to loss of organic matter via decomposition, which was 24% on average (Hofstede, 1994). Table I. Total metal content in MSW compost (mglkg dry matter) Metals.], Spiked RM%~
Initil concentration 10% I 20% I 30% Control I Blank
After 2 J days compostmg 10% I 20% I 30%
Controll Blank
1.3±O.31 38±o.3 SI±O,61 73±l.7 I 74±O,1 4.2±O,21 S2±O,7 SO Cr , . .. Note: ± number IS standarddeviation; RM % - percentage of red mud addition,
-
73±O,61 82±O.1 1100±O.1
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 I kg red mud (dry). Based on dry matter weight, the 10% red mud addition was equivalent to 1/(4+1) 20% red mud, (similarly for the 20% (wet weight) is 2/6=33% (dry weight) and for 30% is 3n 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 composting, and compared to the measured value (Table 2). The metal content of red mud is shown in Fig. 3.
=
=
o. HO and L. QIAO
20
Table 2 Total metal content in compost (mglkg); Comparison between measured and calculated values Metals,J.
Spiked
RM%-t Cr Note:
After 20 days composting
At beginning
10% I 10%*' 20% I 20%*1 30% I 30%* 10% I 10%*' 20% I 20%*' 30% I 30%* . 51 I 77 I 73 I 104 I 74 I 121 73.3 I 92 I 82 I 1141 100 I 131 SO .. RM% percentage of red mud addition; * calculated values (see text)
=
=
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). Leachability of heavy 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.
....
-..r
4
11
3
..,
2
o
0 day of compostlng
~ 20th day of compostlng
to)
0
~ ~ ~ ~ ~
u
Red Mud Additlon (%) Figure I. The leachable metal con lent in municipal compos!. 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 composting 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 ayailability 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.
21
Chromium speciation in municipal solid waste
o
8
0 day of cempesting
6
C r ( m g/ k g)
~ 20 th day of cempest i ng
4 2
o ...jl-.JajL.,LContr 01 Blank 10% 20 % 30 %
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 specjation 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 digest ion. and countercurrent washing) and has very little organic matter associated with it. . 230
33.3
7.8
21.9
20.1
100
#
= T otal me tal ( rug/k g)
80 %
Exchangeable
0 60
[J Ca rbo nates I§ Iron o xides bound
40
ill 0
20 0 Cr
OJ
Ni
Pb
Zn
Or gan ic frac tion Resid ue
o detect ab le Cd in red mud .
Metal s 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 con figuration closest to a noble gas with a high spherical symmetry and the lowest polarisability among the six tested metal s. 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 spik ing metal solution not only increased the exchangeable Cr (as with the other metals). but also increased the Cr bound to organic and oxides in the blank MSW more than was observed with other metals. The Cr spec iation in the control samples may have had large analytical errors due to the low Cr content «1.5 mg/kg).
22
G. HO and L. QIAO
The compo sting process did affect the spec iation of Cr in the compost though the magnitude of chang es was not marked. The carbonates and oxides bound Cr were converted into the organ ic bound fraction during the compo sting process as the result of the competition of Cr with other metal cations for the limited humic organ ic ligands produced during MSW composting (Fig. 4). Th is increase was slowed down by red mud addition. The effect of the composting proces s on Cr specia tion was similar to that with biosolids composting (Qiao, 1996). This conversion increased the stabi lity of Cr complexes in the mature biosolids compost. Like the Cr in biosolids (Qiao and Ho 1997), the speciation of Cr in the MSW compost was chang ed by red mud amendment due to the high Cr concentrat ion 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 recove ry rate (6 1%) of total Cr from the compost. About 40% of the total Cr was conve rted 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 env ironmental cond itions, much of it was unlikely to be released . 1.4"
39
49
52
64
1.5
100
58
66
74
88 "' lotal Cr (mglk g)
80
..
~
60
U
0
exchangeable
81
carbonates
m iron oxides bound
40
m organic fraction
20
residual
0
~
eC 0
U
:§:
~
~
~
~
~
;I
~
c:
01
iii
0
0 N
0 M
e e e e e
eC 0
~
c:
01
iii
;I
e
~
0 N
;I 0 M
U
Red mud add Ilion (a) .- At beginning 01 compo sting (b) ••• After composting lor 20 days Control ··· no red mud and no spiking with metal solut ion Blank ••• no red mud, but spiked with meta l solut ion Figure 4. D istribut ion of Cr in MSW compost.
Correlation between leachable. plant ava ilable metals and metal speciation The relationship between plant available metal and the metal in exchangeable , carbonate and bound-tooxides 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 2 for the extraction of exchangeable metal ( I M) has a higher concentration than the reagent CaCI Z used for the extraction of leachable metal (0.0 1 M), the exchangeable metal concentration is higher than the leachable metal concent ration. The metals in the excha ngeable form may leach from the waste under certain condit ion such as acidic rain, or saline wastewater irrigation.
23
Chromiumspeciation in municipalsolid waste 50
CD
40
!
30
~ III
] :!: ~
500
CD
~ E '-'
20
flJ
DTP A
•
Sum
250
] :!: ~
10 0 Qj
Cu·
Cr
NI
Pb
0
zn
Metal
Metal
Figure 5. Comparison of metal extractedby DTPA with the metals in soluble and weakly bound fractionsin the MSW compost. Note: Sum Sum of exchangeableand 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 (RN03 + HCI04 + HCI). 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 Hofstede, H. (1994). Use of bauxiterefining residue to reduce the mobilityof heavymetals in municipal solid wastecompost. 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 metals in the Environment. VoL I: Heavy Metals in the Environment. J.- P. Vernet (ed), Elsevier, Amsterdam,pp. 67-94. Petruzzelli,G. (1989). Recyclingwastes in agriculture: heavy metal bioavai1ability, 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 of 9th International Conference on HeavyMetals in the Environment, Toronto, Canada, 2, 450-453. Qiao, L. (1996). The mobilityof heavy metals in clay amended sewage sludgeand municipal solid waste compost. Ph D thesis, 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. Water Research, 31,951-964 . Tessier, A., Campbell, P. G. C. and Bisson, M. (1979). Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry, !II, 844-850.
Pergamon
War. Sci. Tech. Vol. 38. No.2, pp. 17-23, 1998. IAWQ C 1998 Published by Elsevier Science Ltd. Printed in Great Britain. All rightJ reserved
PH: S0273-1223(98)00427-2
0273-1223/98 $19'00 + 0-00
CHROMIUM SPECIATION IN MUNICIPAL SOLID WASTE: EFFECTS OF CLAY AMENDMENT AND COMPOSTING Goen Ho and Liang Qiao Institute/or Environmental Science. Murdoch University, Murdoch 6150. Western Australia
ABSTRACf 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 rnaner, 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. ~ 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Bauxite refining residue (red mud); chromium; composting; metal mobility; metal speciation; municipal solid waste. INTRODUCTION Compost produced from mixed MSW can be contaminated by heavy metals derived from contamination of domestic waste by dry cell batteries, metal coatings, paints, solvents, cosmetics, dyes, pesticides, lubricants and metals in electronic equipment and other discarded domestic appliances. One way of overcoming the potential heavy metal problem of compost produced from mixed MSW is to add clay particles « 2 urn) which have a large surface area and capacity to adsorb heavy metals, thus reducing the leachability and plant availability of heavy metals in the compost. Hofstede (1994) investigated the immobilisation of heavy metals in the compost of artificial MSW using bauxite refining residue (red mud) amendment. A mixture of grass clippings and sawdust was spiked with a metal solution to make artificial MSW, amended with red mud and then composted in controlled laboratory 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 CaCI2 and DTPA extraction and acid digestion (HN0 3, HCI0 4 and HCI) 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 AI 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 AI 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): lOmglkg cadmium (Cd(N0 3h ), 50 mglkg chromium (CrCI306H20), 50 mglkg lead (Pb(N0 3h), 20 mglkg nickel (Ni(N03ho6H20), 100 mglkg copper (CuS0405H20) and 500 mglkg zinc, (ZnS0407H20). The mixtures were composted for 20 days, after which samples of MSW compost were dried at lO5°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 I gram samples (based on dry matter) were employed for the metal extraction. A sequential step extract ion was carried out employing 1M MgCI2 (exchangeable fraction); 1M HOAclNaOAc at pH 5 (carbonate); 0.04 M NH20HHCI at 96°C (reducible or bound to oxides); 30% H202/3.2 M NH40Ac at pH 2 and 85'C (bound to organic matter) extractions and concentrated nitric,
Chromiumspeciationin municipal solid waste
19
perchloric and hydrochloric acid digestion (residue fraction) (Tessier et 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 CaCI2 and 0.1 M DTPA followed by an acid digestion (HN0 3-HCI04-HCI) 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 RC5B ultra centrifuge at 10,000 rpm for 20 minutes. The supernatant was passed through a GF/C 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 composting 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 composting process concentrated the total metal in the compost due to loss of organic matter via decomposition, which was 24% on average (Hofstede, 1994). Table I. Total metal content in MSW compost (mglkg dry matter) Metals.], Spiked RM%~
Initil concentration 10% I 20% I 30% Control I Blank
After 2 J days compostmg 10% I 20% I 30%
Controll Blank
1.3±O.31 38±o.3 SI±O,61 73±l.7 I 74±O,1 4.2±O,21 S2±O,7 SO Cr , . .. Note: ± number IS standarddeviation; RM % - percentage of red mud addition,
-
73±O,61 82±O.1 1100±O.1
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 I kg red mud (dry). Based on dry matter weight, the 10% red mud addition was equivalent to 1/(4+1) 20% red mud, (similarly for the 20% (wet weight) is 2/6=33% (dry weight) and for 30% is 3n 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 composting, and compared to the measured value (Table 2). The metal content of red mud is shown in Fig. 3.
=
=
o. HO and L. QIAO
20
Table 2 Total metal content in compost (mglkg); Comparison between measured and calculated values Metals,J.
Spiked
RM%-t Cr Note:
After 20 days composting
At beginning
10% I 10%*' 20% I 20%*1 30% I 30%* 10% I 10%*' 20% I 20%*' 30% I 30%* . 51 I 77 I 73 I 104 I 74 I 121 73.3 I 92 I 82 I 1141 100 I 131 SO .. RM% percentage of red mud addition; * calculated values (see text)
=
=
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). Leachability of heavy 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.
....
-..r
4
11
3
..,
2
o
0 day of compostlng
~ 20th day of compostlng
to)
0
~ ~ ~ ~ ~
u
Red Mud Additlon (%) Figure I. The leachable metal con lent in municipal compos!. 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 composting 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 ayailability 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.
21
Chromium speciation in municipal solid waste
o
8
0 day of cempesting
6
C r ( m g/ k g)
~ 20 th day of cempest i ng
4 2
o ...jl-.JajL.,LContr 01 Blank 10% 20 % 30 %
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 specjation 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 digest ion. and countercurrent washing) and has very little organic matter associated with it. . 230
33.3
7.8
21.9
20.1
100
#
= T otal me tal ( rug/k g)
80 %
Exchangeable
0 60
[J Ca rbo nates I§ Iron o xides bound
40
ill 0
20 0 Cr
OJ
Ni
Pb
Zn
Or gan ic frac tion Resid ue
o detect ab le Cd in red mud .
Metal s 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 con figuration closest to a noble gas with a high spherical symmetry and the lowest polarisability among the six tested metal s. 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 spik ing metal solution not only increased the exchangeable Cr (as with the other metals). but also increased the Cr bound to organic and oxides in the blank MSW more than was observed with other metals. The Cr spec iation in the control samples may have had large analytical errors due to the low Cr content «1.5 mg/kg).
22
G. HO and L. QIAO
The compo sting process did affect the spec iation of Cr in the compost though the magnitude of chang es was not marked. The carbonates and oxides bound Cr were converted into the organ ic bound fraction during the compo sting process as the result of the competition of Cr with other metal cations for the limited humic organ ic ligands produced during MSW composting (Fig. 4). Th is increase was slowed down by red mud addition. The effect of the composting proces s on Cr specia tion was similar to that with biosolids composting (Qiao, 1996). This conversion increased the stabi lity of Cr complexes in the mature biosolids compost. Like the Cr in biosolids (Qiao and Ho 1997), the speciation of Cr in the MSW compost was chang ed by red mud amendment due to the high Cr concentrat ion 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 recove ry rate (6 1%) of total Cr from the compost. About 40% of the total Cr was conve rted 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 env ironmental cond itions, much of it was unlikely to be released . 1.4"
39
49
52
64
1.5
100
58
66
74
88 "' lotal Cr (mglk g)
80
..
~
60
U
0
exchangeable
81
carbonates
m iron oxides bound
40
m organic fraction
20
residual
0
~
eC 0
U
:§:
~
~
~
~
~
;I
~
c:
01
iii
0
0 N
0 M
e e e e e
eC 0
~
c:
01
iii
;I
e
~
0 N
;I 0 M
U
Red mud add Ilion (a) .- At beginning 01 compo sting (b) ••• After composting lor 20 days Control ··· no red mud and no spiking with metal solut ion Blank ••• no red mud, but spiked with meta l solut ion Figure 4. D istribut ion of Cr in MSW compost.
Correlation between leachable. plant ava ilable metals and metal speciation The relationship between plant available metal and the metal in exchangeable , carbonate and bound-tooxides 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 2 for the extraction of exchangeable metal ( I M) has a higher concentration than the reagent CaCI Z used for the extraction of leachable metal (0.0 1 M), the exchangeable metal concentration is higher than the leachable metal concent ration. The metals in the excha ngeable form may leach from the waste under certain condit ion such as acidic rain, or saline wastewater irrigation.
23
Chromiumspeciation in municipalsolid waste 50
CD
40
!
30
~ III
] :!: ~
500
CD
~ E '-'
20
flJ
DTP A
•
Sum
250
] :!: ~
10 0 Qj
Cu·
Cr
NI
Pb
0
zn
Metal
Metal
Figure 5. Comparison of metal extractedby DTPA with the metals in soluble and weakly bound fractionsin the MSW compost. Note: Sum Sum of exchangeableand 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 (RN03 + HCI04 + HCI). 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 Hofstede, H. (1994). Use of bauxiterefining residue to reduce the mobilityof heavymetals in municipal solid wastecompost. 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 metals in the Environment. VoL I: Heavy Metals in the Environment. J.- P. Vernet (ed), Elsevier, Amsterdam,pp. 67-94. Petruzzelli,G. (1989). Recyclingwastes in agriculture: heavy metal bioavai1ability, 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 of 9th International Conference on HeavyMetals in the Environment, Toronto, Canada, 2, 450-453. Qiao, L. (1996). The mobilityof heavy metals in clay amended sewage sludgeand municipal solid waste compost. Ph D thesis, 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. Water Research, 31,951-964 . Tessier, A., Campbell, P. G. C. and Bisson, M. (1979). Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry, !II, 844-850.