Journal of colloid and interface science article

Journal of colloid and interface science article

Pergamon Wat Sci Tech Vol 38, No 2 pp 17-23, 1998

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0 1998 Published by Elsevier Science Ltd hinted in Great Britain All rights reserved PII: SO273- 1223(98)00427-2 0273-1223/98 $19fKl+ OCG

SOLID WASTE: EFFECTS OF CLAY

Goen Ho and Liang Qiao

Institute for Environmental Science, Murdoch University, Murdoch 6150

Western Australia

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; 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 pm) 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

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,

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)

Metals& Spiked Initial concentration After 20 days composting

RM%+ Control 1 Blank 1 10% 1 20% 1 30% Control1 Blank 10% I 20% I 30%

Cr 50 1.3f0.3 1 38f0.3 1 5tfO.6 1 73fl.7 1 74f0.1 4.2fO.21 ~$0.7 73fo.6 1 &O.l I10&0.1 Note: f number is standard deviation; RM %I = percentage of red mud addition

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

Table 2 Total metal content in compost (mg/kg); Comparison between measured and calculated values

At beginning I After 20 days composting I RM%-B 1 1 10% 1 lo%*I 20% 1 209b’l 30% [309b*l 10% I10%*( 20% I 20%*( 30% 1309P

Cr 1 50 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 Note: 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)

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

82

q 20th day of composting

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

21

0 0 day of cornpasting

6

Cr (w/kg) 4 q 20th day of composting

2

0 Control 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 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

# = Total metal OwYW

q Exchangeable

q Carbonates

H Iron oxides bound

q Organic fraction Residue

Metals

No detectable Cd in red mud

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

increased the Cr bound to organic and oxides in the blank MSW more than was observed 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)

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

t

q carbonates

40 q iron oxides bound

III organic fraction

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

23

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

Hofstede H (1994) Use of bauxite refining residue to reduce the mobility of heavy mefals in municipal 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 of 9th International Conference on Heavy Metals in the Environment, Toronto, Canada, 2, 450-453

Qiao, L (1996) The mobility of heavy metals in clay amended sewage sludge and 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 Warer

Research, 31.951-964

Tessier, A., Campbell, P G C and Bisson, M (1979) Sequential extraction procedun for the speciation of particulate trace