Removal of heavy metals from compost and soil by ecotechnological methods

Ecological Engineering, 2 (1993) 89-100 Elsevier Science Publishers B.V., Amsterdam

89

Removal of heavy metals from compost and soil by ecotechnological methods Sven Erik J0rgensen DFH, Institute A, Environmental Chemistry, University Park 2, 2100 Copenhagen O, Denmark (Received 19 September 1992; accepted 3 February 1993)

ABSTRACT Heavy metal pollution of soil and compost is an important problem in many industrialized countries. Soil purification facilities for removal of heavy metals by use of environmental technological methods have been installed in many countries. These methods are, however, expensive, as they imply removal of the soil, transportation of the soil to the plant and purification at the plant. This paper reports the results of the application of three different ecotechnological methods which can be used in situ or to prevent the soil contamination. The methods were (1) separation of organic waste and other types of household waste, (2) removal of heavy metals by EDTA used in situ, and (3) use of plants and addition of EDTA for removal of heavy metals in situ. All three methods gave promising results, with high removal efficiencies of heavy metals (more than 90% for mercury and copper). The selection of one of the methods is not possible on a general basis, as the choice depends on the pollution level, the distribution of heavy metals, the types and speciation of heavy metals and the soil characteristics. INTRODUCTION T h e use of composting for t r e a t m e n t of garbage is an attractive solution to solid waste problems, because the product may be used as soil conditioner. The compost p r o d u c e d from domestic garbage cannot, however, be applied as soil conditioner in D e n m a r k due to elevated concentrations of heavy metals, mainly lead and cadmium. Heavy metals may be removed by extraction m e t h o d s at the composting plant, but an alternative would be to sort the garbage into wet (green) and dry fractions at the source. This sorting may be considered an ecotechnological solution to the problem. The standards used in D e n m a r k are shown in Table 1, where the concentrations of heavy metals in compost p r o d u c e d from unsorted and sorted

Correspondence to: S.E. Jorgensen, DFH, Institute A, Environmental Chemistry, University Park 2, 2100 Copenhagen 0, Denmark. Elsevier Science Publishers B.V.

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garbage (the wet fraction) are compared. Furthermore, the table gives an analysis of typical Danish soil. The results of a pilot plant examination of heavy-metal concentrations in sorted wet waste and in the final compost are reported in this paper. The complex binder E D T A (ethylene-diamine-tetra-acetate) is able to dissolve heavy metals. This ability can be utilized to remove heavy metals in situ, and to collect the E D T A - h e a v y - m e t a l soltition drained from the field. An investigation on a 100-m 2 plot contaminated with lead will be presented below. A third solution to the problem of heavy-metal contamination is examined. Green plants are able to take up metal ions from soil water. Fortunately only a minor fraction of the heavy metal is soluble, depending on the characteristics of the soil (see JCrgensen, 1975, 1976, 1988). If the plants are watered with E D T A solutions, it is possible to dissolve a significant fraction of the heavy metals and a high concentration of heavy metals in the plants is achieved. The heavy metals are consequently removed from the soil by harvesting the plants. Greenhouse pot experiments based upon this method have been carried out and the results are reported below. After presentation of the results of the three alternative methods for heavy-metal removal, the methods will be compared and their feasibility will be discussed. General guidelines on where to use each of the three methods cannot be given as none of the methods is superior to the others under all conditions. ECOTECHNOLOGY APPROACHES TO REMOVING HEAVY METALS

Sorting household garbage The experiment was carried out by the Community of Copenhagen and "Renholdningsselskabet af 1898", while the examinations were undertaken by the consulting engineering firm "Birger Lund". Fifteen h u n d r e d households participated voluntarily in the experiment, where each household had to sort the garbage into two fractions: a wet fraction (mainly from the kitchen) in a green bag and a dry fraction of other waste in a red bag (bottles and to a certain extent metals are recycled in Denmark). About 1200 kg of the wet waste was collected per week. This was composted in an experimental composting unit, after addition of 10% straw to 90% wet waste. The first week of the composting process was carried out in an aerated container, followed by 8-10 weeks composting in piles. The compost was screened on a 20-mm sieve before use. The coarse fraction was incinerated, while the final fraction was used as soil conditioner.

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91

TABLE 1 Metal concentrations and ranges (unit: m g / k g dry matter) in compost from sorted and unsorted garbage

Metal

Wet waste mean, r a n g e in brackets (n = 1500)

Cd Hg Pb Cr Ni Cu As

0.1 (0.06-0.25) 0.1 (0.04-0.20) 8.0 (2.6-24) 1.6 (0.3-3.7) 1.2 (0.6-1.9) 14 (10-16) 0.5 (0.3-0.9)

Sorted Unsorted compostp r o compostproduced from duced from wet waste garbage (n = 1500) 0.3 (0.26-0.43) 1.3-5.5 0.2 (0.11-0.28) 1.3-2.4 21 ( 1 1 - 6 0 ) 131-745 43 (18-79) 70-80 10 (7-12) 45 40 ( 2 9 - 4 9 ) 250-300 1.4 (0.3-2.1) 5.2

S t a n d a r d s Mean in Densoil mark Denmark 0.8 0.8 120 a 30 25

0.1 0.1 20 18 3 22 1.1

a 80 mg/kg when applied in private gardens.

Methods Twelve random samples of 100 kg each (fresh weight) were taken from the wet waste during a period of 6 months. Each sample consists of many random sub-samples. A 25-g sample was used for analysis of heavy metals (Cd, Pb, Hg, Ni, Cu, Cr, and As~ by AAS (Atomic Absorption Spectrophotometry).

Results The mean concentrations of heavy metals and As in the wet fraction and in the final compost are summarized in Table 1, where the ranges are also given. The wet waste has a high variability due to heterogeneity, while the variations are more moderate in the final compost due to a better mixing of the final product. The Danish Standards for the application of compost and sludge as soil conditioner, the results obtained for compost from unsorted garbage (from a previous examination, Birgerlund, pers. commun.) and the average analysis of Danish agricultural soil are all given in the table for comparison. Determinations of the dry matter and the ash content of the wet waste and the final compost after screening has revealed that 26% of the original dry matter is found in the compost and 4% in the sieve residue. The heavy-metal concentrations of the sieve residue and the straw applied as conditioner were determined in order to set u p a mass balance for the various heavy metals. These results given in Fig. 1 indicate that there is a reasonable accordance of the mass balance for lead, copper, arsenic and cadmium, while the mass balances for nickel and chromium and to a minor extent for mercury show discrepancies.

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S.E. JORGENSEN

BASIS: I00 KG DRY MATTER. Wet Waste 80 kg Ash 8.5 kg Pb 640 mg Cd 8 m g

Hg As co Cr

Straw 20 kg Ash I kg Pb 48 mg Contamination

8 mg ~ 38 m g ~ 112o 128 mg ~

Ni 96 m g ~

Ni5mg

I

I

Cd 2.2 m g

Cr llOSmg? HgO.6mg Ni 202 mg? I As 8 mg I I Cu 46 mg ~ ~ J C r II mg

COMPOSTINGPROCESSES

SCREENING(20

kg Ash 1.4 kg PIb 344 mg Cd 1.9 mg Hg 1.0 mg As 7.0 mg Cu 312 mg Cr 128 mg Residue 4

Ni 56 mg

mm Sieve)

I

I

Final compost Ash 7.1 kg

Pb 546 mg Cd 8.8 mg Hg 5.2 mg As 36 mg Cu 1040

Cr 1118 mg Ni 247 mg

Evaporation: 2.4 mg Hg ?

Fig. 1. Mass balances for metals and ash after composting of 100 kg sorted waste (dry matter).

The higher concentrations in compost compared with wet waste as shown in Table 1 can be easily explained by the decomposition of organic matter which causes a higher concentration of conservative components. The significant increase of nickel and chromium beyond the mass balance may have been due to contamination during the composting process or to disintegration of the samples before analysis. The composting tank and the disintegrator were made of steel with a certain content of nickel and chromium. The minor decrease of mercury components during the composting process could probably be explained by evaporation of organic mercury compounds, as the temperature may rise to 50-60°C. The investigations have shown that it is possible to obtain sufficiently low heavy-metal and arsenic concentrations in the final compost based

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R E M O V A L OF HEAVY METALS FROM COMPOST

upon the wet fraction of domestic garbage to meet the Danish Standards, which may be considered rather low (severe). Sorting is therefore sufficient to produce a compost that can be used safely in gardens and in agriculture. It seems to be most difficult to meet the standards for cadmium and lead. The results demonstrate that diffuse sources of heavy-metal pollution are significant, since the major portion of the point sources should have been removed by sorting the garbage. The method is recommendable provided that a proper sorting of the garbage can be assured. In situ E D T A extraction

E D T A forms very stable complexes with most heavy metals (see Miljcstyrelsen, 1990). The formation constant is rather large, but side reactions for E D T A and the metal ions play a role, too. Therefore the conditional equilibrium constant, K *, is used which is defined as: K * = K × tim

× ~EDTA

where K is the equilibrium constant for formation of the complex metalEDTA,/3,, is the concentration of free metal ion relative to the total metal concentration, including metal hydroxides and hydroxo-complexes and //Er,TA is the concentration of E D T A without the four hydrogen ions (the form which reacts with the metal ions). K * is dependent on pH, as/3,, and ~EDTA vary with pH due to the side reactions. K * times the metal-ion concentration as a function of pH for lead and cadmium (the two heavy metals which cause the largest problems) is shown in Fig. 2. Figure 3 gives

log

K* [ M e 2 + ]

,",

18

¢

log K*tCd] IogK*[Pb]

16 14 12 10 8 6 4 2

4

!

I

6

8

I0

12

pH Fig. 2. L~g K * (lead or cadmium conditionalequilibriumconstant)versus pH.

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S.E. JORGENSEN

log [EDTA-metal ion] (solubility) 12 t0

~-

[EDTA-Cd]

•

[EDTA-Pb]

8 6 4 2 0

.

2

.

4

.

.

6

.

8

10

12

14

pH Fig. 3. The solubility of E D T A - I e a d and E D T A - c a d m i u m complexes versus pH.

the hypothetical solubility of metal-ion-EDTA for lead and cadmium if a 10% stoichiometric surplus is added to the soil, sludge or compost. As seen from the last figure, a pH of around 5.0 gives the most favorable conditions for the dissolution and removal of these heavy metals. Lower pH gives lower values of flEDTAand higher pH gives a lower solubility of metal ions. Methods The application of E D T A in situ was tested on a plot which was contaminated by lead because it was used as shooting range for several decades. Figure 4 shows a profile of the lead concentration in a characteristic soil sample of the plot. Ten samples with different lead concentrations were treated with different E D T A amounts/concentrations before the pilot plant experiment in the laboratory. The results of the laboratory investigations are summarized in Table 2. It is clear from these preliminary examinations that a significant amount of lead can be removed from the soil by E D T A at a pH of about 5.0. Regeneration of the E D T A - l e a d solution has also been tested in the laboratory. Sodium hydroxide is added until pH = 8.0-8.5. Most of the dissolved lead is precipitated at this pH as lead-hydroxide, leaving the E D T A in solution. The lead still in solution may be removed by treatment of the solution on an ion exchanger produced by treatment of spruce or pine bark by sulfuric acid. (This is a commercially available product that is highly selective to heavy metals.) The lead concentration after precipitation was 7.6 mg/1 and after ion exchange was 0.6 mg/1 from an initial extracted concentration of 345 mg/1.

REMOVAL

OF HEAVY

METALS FROM

95

COMPOST

Lead concentration, mg / kg dry matter 0

0

~

100

200

30

80f

300

400

~-r-r-r-r-r-r-r-r-r~0 v

80 Depth, cm

Fig. 4. Profile of lead concentrations versus soil depth in contaminated soil to be treated by EDTA in situ method.

T h e s o l u t i o n a f t e r t h e ion e x c h a n g e p r o c e s s c a n b e r e u s e d a f t e r a d j u s t m e n t o f p H to a b o u t 5.0, w h e r e t h e d i s s o l u t i o n o f l e a d is m o s t effective. A f l o w c h a r t o f t h e c h a i n o f p r o c e s s e s is s h o w n in Fig. 5. T h e r e u s e o f t h e E D T A s o l u t i o n w a s t e s t e d twice o n a l a b o r a t o r y scale. O n e h u n d r e d m 2 o f a l e a d - c o n t a m i n a t e d p l o t w a s t r e a t e d w i t h E D T A at p H -- 5.0. D r a i n a g e p i p e s w e r e p l a c e d at a d e p t h o f 0.7 m. I r r i g a t i o n b y t h e u s e o f 140 m 3 E D T A s o l u t i o n w i t h a c o n c e n t r a t i o n o f 2 g / 1 o v e r a p e r i o d

TABLE 2 Extraction of lead by EDTA (applied as 20 mmol solution per kg of soil) from soil samples. The numbers given are mg Pb/kg soil removed (pH -- 5.0) Pb in sample (mg/kg)

g E D T A / k g soil 1 5

10

20

30

% removed with 10 g EDTA/kg soil

123 222 276 300 412 445 533 780 822 990

34 45 44 65 78 67 45 79 109 120

99 187 213 236 356 400 453 695 767 823

101 196 211 245 371 392 476 702 768 845

100 192 218 251 360 396 480 654 790 809

80 84 77 79 86 90 85 89 93 83

56 100 127 145 176 169 209 285 565 569

Only slightly higher efficiencies were obtained above 10 g E D T A / k g soil. Thefefore, an application of 10 g E D T A / k g soil was used in the calculations of % removable.

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s.E. JORGENSEN

EDTA, added I

Treatment of soil

~ Drainagewatlr ~ Reuse of EDTA

I

Na OH ~ Precipitation of heavy "1 metal ions

~

I[

Adjustment

,~ Heavy metal hydroxides

Solution

Ion exchange

~

lncinerated, when saturated

Fig. 5. The chain of processes used for removal of heavy metals from soil in situ by the use of E D T A . of 10 days (2 weeks of 5 days each, 7 h per day) was performed. The concentration of lead was measured for the drainage water as a function of the irrigation time. The flow of drainage water was measured regularly (3 times a day or almost every second hour). Results The results are shown in Fig. 6. A n integration of the flow rates over the entire time of operation gives a 96% recovery of the irrigation water. By integrating Figs. 4 and 6 it is possible to calculate the efficiency of the Pb mg/I

200'

100

0 0

20

40 (h)

60

80

Time

Fig. 6. Concentration of lead (mg/l) in drainage water as function of the treatment time for in situ EDTA extraction. 0.02 M EDTA was used.

97

REMOVALOF HEAVYMETALSFROM COMPOST

extraction process. Figure 4 gives the amount of lead in the soil and Fig. 6 gives the amount of lead removed by the process. The calculation indicates that 82% of the lead can be removed. Figure 7 illustrates a mass balance calculated on the basis of the regeneration experiments carried out in the laboratory and the results of the pilot plant experiment for the extraction of lead in situ. The results of this method applied in situ are very promising. An 87% removal of lead means that the upper 30 cm, where lead concentration averages 380 m g / k g can be reduced to 49 mg/kg. This is slightly more than twice the average for Danish soils, but may also be compared with the standards of 80-120 m g / k g for sludge and compost. The environmental authorities have accepted this lead level for a normal utilization of a plot. It would, however, be interesting to test the methods on other lead-contaminated plots, as it cannot be excluded that the lead compounds on this plot were particularly easily removed by EDTA. A wider test of the method, including its applicability on other heavy metals, is strongly recommended before a more general conclusion on the applicability of the method can be made.

Plants and in situ application of EDTA The method based upon extraction of heavy metals from soil in situ requires only that a tiny portion of the drainage water might be lost to

J

2 m 3 solution (4 kg EDTA)

About 140 g Pb 18 g remains i I~ 122 g Pb is removed

12 kg NsOH

107 g Pb is removed as Pb(OH)2

J

-I I

Precipitation

lon exchange with a Cape. clty of 3 0 g / l

EDTA for reuse, conI talns 1.2 g Pb (0.6 mg/l)

I

'~

Fig. 7. Lead balance based upon the pilot plant experiments and the laboratory investigations of the possibilities to recover the E D T A solution. 1 m 2 of the plot (depth 0.7 m) is considered.

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S.E.JORGENSEN

TABLE 3 Results of pot experiments for lead removal using green plants and EDTA. Pb concentrations are in mg/kg dry matter (range in parentheses). Experiments A and B are with normal Danish soil, while C and D are with soil from a shooting range. Experiments A and C: no addition of EDTA. Experiments B and D: a 0.02 M EDTA solution was used Experiment

A (no EDTA) B (0.02 M EDTA) C (no EDTA) D (0.02 M EDTA)

Pb concentrations (mg/kg dry matter) Soil Roots Plant (stem + leaves) 22 1.2 (0.2-1.9) 2.0 (1.2-4.5) 22 11 (8-17) 20 (12-41) 480 44 (32-60) 135 (95-162) 480 231 (203-288) 490 (410-546)

groundwater. Furthermore, the installation cost of drainage pipes is significant, although the m e t h o d is much more cost effective than an ex situ treatment. It would therefore be attractive if the dissolved lead after in situ treatment by E D T A could be removed by another more cost-effective method. Experiments in pots were therefore p e r f o r m e d to observe w h e t h e r green plants w a t e r e d by E D T A solution would be able to take up enough lead that it would be possible to remove the heavy metals by harvesting the plants over a period of a few years.

Methods Ten pots had soil with low (normal) lead concentration and ten other pots had soil from the shooting range. Five of the ten pots in each group were w a t e r e d by 0.02 M E D T A . Calcium oxide was a d d e d to all experiments. C o m m o n maize was used in this case as a plant.

Results The results show that it is possible to remove lead by the use of plants w a t e r e d by E D T A solutions (Table 3). As the plants harvested had an average dry weight of 113 g / k g of soil, 11.5% was removed. Therefore a substantial part of the lead can be removed by this method. It is, of course, not possible to give any information on the a m o u n t of lead that can be removed by a r e p e a t e d use of the method, but it cannot be excluded that it is possible to remove sufficient lead by 6 - 1 0 harvests. If that is possible, the m e t h o d would be significantly c h e a p e r than ex situ purification of any form and c h e a p e r than the use of drainage pipes. The plants can be incinerated after harvest, which would p r o d u c e fly-ash and slags with elevated concentrations of lead. The amount of fly-ash and slag from incineration of the

REMOVAL OF HEAVY METALS FROM COMPOST

99

plants are, however, very limited and the deposition problem is not different from the corresponding problem for fly-ash and slags originating from garbage. DISCUSSION Three different methods for removal of heavy metals from compost and soil have been investigated. The first method is based upon sorting at the source and may be used to avoid the problem of too elevated heavy-metal concentrations in compost produced from garbage and used as a soil conditioner in agriculture. The two other methods are used in situ for removal of heavy metals from an already contaminated soil. All three methods are based upon ecological engineering principles (see Jergensen and Mitsch, 1989; Mitsch and Jergensen, 1989). The application of sorting at the source follows principle number three: Elements are recycled in ecosystems. Matching humanity and ecosystems in recycling pathways will ultimately reduce the effects of pollution. The second method, where the lead is removed by extraction processes, follows principle number four: Homeostasis of ecosystems requires accordance between biological function and chemical composition. The chemical composition in this case is corrected by the use of EDTA. The third method, using plants for removal of the surplus lead, is based on principle number two: Ecosystems are self-designing systems. The more one works with the self-designing ability of nature, the lower the costs of energy to maintain the system. Plants can remove dissolved heavy metals, which are the most toxic parts of total heavy-metal concentrations. This self-designing ability is reinforced by adding EDTA, because E D T A increases the water solubility of heavy metals. The E D T A complexes of heavy metals are much less toxic than the metal ions. An economic analysis of the three methods has not been carried out, but all three are rather low-cost. The results of the three methods are sufficiently attractive to encourage a wider application on a bigger scale. Sorting garbage at the source is already used on a large scale in Denmark with at least partial success. For decontamination of soil, the use of plants and the addition of E D T A seems particularly attractive, because it is easy and inexpensive to perform. It is important to keep the EDTA-heavy-metal solutions from moving toward the groundwater. Either a membrane or a controlled watering scheme according to the soil humidity might solve this problem properly, but further experiments on a larger scale are needed before a practical solution is available.

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S.E.JORGENSEN

CONCLUSIONS Three ecological engineering m e t h o d s for a reduction of heavy-metal concentrations in compost and soil have b e e n examined and the results obtained look very promising as all three methods can offer a solution. Sorting at the source makes it possible to obtain sufficiently low heavymetal concentrations in compost to allow the use of compost as soil conditioner in agriculture. The compost meets the standard used in Denmark. Watering with E D T A solution in a pilot plant experiment (100 m 2 were used for this experiment) showed that it is possible to remove more than 80% of the lead from a plot that has b e e n used as shooting range. It is of course not possible to generalize from these results to other types of lead contamination or to other heavy metals. The removal of heavy metals by plants and E D T A offers a cost-effective solution, based on sound ecological engineering principles, although further investigation of the methods on a larger scale is absolutely necessary before a final conclusion can be made. M o r e than 11% of lead was removed by this m e t h o d in one harvest, but five or more subsequent harvests should be tested. It is planned for the near future to perform such a long-term test of the method. The ecological engineering methods for heavy-metal removal look very promising and further investigations of the m e t h o d s should be attractive. REFERENCES J~rgensen, S.E., 1975. Do heavy metals prevent the agricultural use of municipal sludge. Water Res., 9: 163-170. J~rgensen, S.E., 1976. An ecological model for heavy metal contamination of crops and ground water. Ecol. Modelling, 2: 59-67. Jc)rgensen, S.E., 1988. Modelling the contamination of agricultural products by lead and cadmium. In: A. Marani (Ed.), Advances in Environmental Modelling. Elsevier, Amsterdam, pp. 343-350. J~rgensen, S.E. and W.J. Mitsch, 1989. Ecological engineering principles. In: W.J. Mitsch and S.E. Jc)rgensen (Eds.), Ecotechnology, An Introduction to Ecological Engineering. John Wiley, New York, NY, pp. 39-56. Milj~styrelsen, 1990. Removal of heavy metal from sludge, compost and soil. Miljcstyrelsen, Copenhagen, Denmark. Mitsch, W.J. and S.E. J¢rgensen, 1989. Introduction to ecological engineering. In: W.J. Mitsch and S.E. Jcrgensen (Eds.), Ecotechnology, An Introduction to Ecological Engineering. John Wiley, New York, NY, pp. 3-12.