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