Heavy metal binding to digested sludge

Water Res. Vol. 19, No. 12, pp. 1483-1488, 1985 Printed in Great Britain. All rights reserved

0043-1354/85 $3.00+0.00 Copyright © 1985 Pergamon Press Ltd

HEAVY METAL BINDING TO DIGESTED SLUDGE K. R. K. ALIBHAI l, I. MEHROTRA 2 and C. F. FORSTER l IDepartment of Civil Engineering, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, England and ~Department of Civil Engineering, University of Roorkee, Roorkee, 247667, India (Received October 1984)

Abstract--The interactions of heavy metals with anaerobically digested sludges and with digested sludge which had been washed with EDTA have been examined by equilibrating the sludge solids with solutions containing zinc acetate, lead acetate, ferrous sulphate and chromic chloride. This equilibration has been studied at 20 and 35°C. Langmuir and Freundlich isotherms were used to describe the metal binding behaviour of the solids. The linear part of the isotherm was obtained by regression analysis and conditional adsorption constants were calculated from the slope of these lines. These calculations suggested that, under the conditions used in this work, the mechanism of attachment was predominantly one involving metal/surface ligand interactions. Attachment series, based on the various constants, are reported and lead the authors to question the suitability of such series for describing the binding of metals relative to one another. Key w o r d s ~ i g e s t e d sludge, heavy metals, adsorption, isotherms, sorption constants, binding series

INTRODUCTION The equilibration, fractionation, characterization and speciation of heavy metals in sewage sludges has been studied extensively. Lake et al. (1984), for example, have reviewed the role and function of metal ions in activated sludges and sludge amended soils whilst Rivera (1983) has discussed the behaviour of zinc in an anaerobic upflow reactor. In anaerobically digested sludge, a major mechanism of heavy metal binding is thought to be chemisorption by weakly acidic functional groups (Tan et al., 1971). The behaviour of these sludges is, therefore, similar to that reported for the organic matter in soil (Lewis and Broadbent, 1961; Schnitzer and Skinner, 1965). The equilibrium between b o u n d and soluble metal ions, in the case of anaerobically digested sludges, is affected by p H and the presence of other competing ions, thus suggesting an ion exchange mechanism (Gould and Genetelli, 1978a, b, 1984; Hayes and Theis, 1978). Although metal binding to the various specific types of sludge has been examined previously, the range of metals examined in any one investigation has usually been restricted. It is, therefore, necessary to extend the range of metals. Also, there is a need to know how metal binding is affected by conditions opposite to those under which the sludge had been produced since this situation can occur in practice. F o r example, when activated sludge is placed in a digester or when digested sludge is disposed of to land. The purpose of this study therefore is to describe the adsorption, under aerobic conditions, of certain heavy metals (Pb, Fe, Zn, Cr) by anaerobically digested sludge. The paper also discusses the significance of "cleaning" the sludge surfaces with

EDTA. This is a technique for removing any metals already present at the sludge surface which would otherwise block adsorption sites (Cheng et al., 1975; Hayes and Theis, 1978).

METHODOLOGY Materials

The sludge samples were obtained from the Bromsgrove sewage treatment works operated by the Severn Trent Authority. The gross characteristics of this sludge are presented in Table 1. The stock solutions of the metal salts (0.05-0.10 M) were prepared and preserved in acidic conditions (0.01% HNO 3 for lead and zinc acetates; 0.01% HC1 for chromic chloride; 0.01% H2SO4 for ferrous sulphate). Metal concentrations were measured by atomic absorption spectrophotometry (Shandon Southern, A3480), total organic carbon (TOC) by the Beckman 915 system and pH values by a standard laboratory meter (EIL, 7020). Procedure

Digested sludge (25 ml) and a solution containing heavy metal salt (75 ml) were mixed for 2 h using a shaker. The temperature was controlled at 20 or 35°C. An aliquot of the original sludge (200 ml) was stirred for 24 h with 4 × 10-4 M EDTA (21.), centrifuged, washed with water and resuspended in distilled water to give a final total volume of 200 ml. The washed sludge (10 ml) was then mixed with the metal salt solution (40 ml) and equilibrated at 20°C. All the experiments were performed at pH 6.8-7.2. Wherever necessary the pH was adjusted to this range using 0.1 M NaOH prior to mixing. The mixtures after equilibration were centrifuged and the residual soluble metal concentrations measured in the supernatant. The solids were then centrifuged with 0.1 N HC1 and the liquid phase from this separation was also analysed for metal ions. In all the cases, the sum of these two metal concentrations accounted for about 97-98% of the metal added. To obtain a spread of results, the metal concentrations of the initial solution ranged from about 0.2 to 4 mmol. The solutions obtained after the equilibrations were also analysed for total organic carbon and inorganic carbon.

1483

K. R. K, ALIBHAI el al.

1484

Fable I, Sludge characteristics Determinant

Digested sludge

pH Alkalinity (rag I ~ as CaCO3/ Total solids (gl i) Volatiles (%) Zinc (mg g ~) Iron (mg g- J) Chromium (mg g-t) Lead (mgg ~)

RESULTS

The data obtained by equilibrating the sludge with the four heavy metals are shown in Figs 1-4. The results are presented as simple isotherms. These show significant differences in the behaviour of the different metal ions and also that washing with EDTA affects the adsorption. The pH measurements, made during the equilibration periods, showed that the acidity did not vary by more than 0.1-0.2 pH units during the tests. The adsorption data were also examined as Langmuir and Freundlich isotherms, the data being fitted by regression analysis using a micro-computer and a standard statistical program. In all but one case (washed sludge and iron) the coefficients of determination were greater than 0.9. In general, the is•-

0.4 -

/....

0.4

f

E E 0.3

03 I

D

0.2 o m

7.2 400 19.02 73 < 1.0 2.3 < 1.0 < 1.0

therms were quite regular (Fig. 5). However, two systems (washed sludge with iron and with chromium) gave irregular Freundlich isotherms (Fig. 5). The isotherm constants, obtained by the regression analyses, are given in Table 2 together with conditional adsorption constants calculated by the technique used by Nelson et aL (1981). The concentration of organic carbon in the supernatant liquors after washing with EDTA was used as an indication of the degree to which the sludge surface had beencleaned. The results (Table 3) show that concentrations up to about 2 x 10-3M achieved a very similar degree of extraction and therefore of surface conditioning. However. the highest concentration that was used (4 x 10 -3 M) produced a significantly higher degree of extraction. This concentration was therefore

0.5

'T

0

Washed sludge

7.2 3000 26.94 64 < 1.0 2.5 < 1.0 < 1.0

~

0.2

~L./' o

0.1

0.1

m ¢ I .I 0.04 0.08 0.12

I

I

I

/

t"

.~

/•

III ° . . . . . . . . . .

I

o- ~

"1

0.16 0 . 2 0 0.24 0.26 0 . 3 2 0.36

0.05

Soluble metal (retool I-~)

/o

I 0.25

0.15

0.35

"/

0.45

0.55

] 0.65

Soluble metot (retool L-~)

Fig. 1. Metal binding curves for d i ~ s t e d sludge. O - - - Q lead at 20 and 35°C; lIt lIt iron at 20°C: • • iron at 35°C.

Fig. 3. Metal binding curves for E D T A treated digested sludge, lIt [] lead; O - - - • iron 1.0--

0.6

/

A

08P-

E

o6-

i

/

---" 0.5

/ /

o~

0.4

E 0.3

0.2 I:~

0.1

#

/ / /

04--

/, . f ' - "

S

02

I

0.03

I 0.06

0.09

i 0.12

J

I

0.15

0.18

0.2

Fig. 2. M e t a l b i n d i n g c u r v e s for d i g i t a l sludge. •

0.4

01.6

0.'8

Soluble metal (mmol 1-1)

Soluble rnetol (mmol 1-1)

c h r o m i u m at 20 and 35°C; I t _ _ - I t

_4"

•

zinc at 20 and 35°C.

Fig. 4. Metal binding curves for E D T A treated digested sludge, m - - - I I zinc; • • chromium.

Heavy metal binding to digested sludge 1.00

1485

1.00

I

°/

I

e/

""

A

ii

•

'T

s

.*/

EE 0.10

..= / a

0.10

Z inc

Chromium

0.01

I 0.10

0.01

I 1.00

0.01 0.01

I 0.10

I

1.0

Ce (retool I-I)

Fig. 5. Freundlich isotherms for digested sludge. • • at 20 and 35°C; • • Data from Gould and Genetelli (1978a). selected for the treatment of the sludges prior to equilibration. DISCUSSION

The results indicate a sorption like behaviour (Figs 1-4) with the binding capacity depending both on the metal and the nature of the sludge. The principal metal forms normally associated with sludges are; soluble, precipitated, co-precipitated with metal oxides, adsorbed and associated with biological residues. The distribution of these forms varies widely according to the chemical properties of the individual metal and the characteristics of the sludge, which, in turn, are a function of the physical and chemical properties imposed by the particular sludge treatment process. The physico-chemical Table

2.

Equilibrium constants for metal/sludge systems Langmuir

System Pb/20°C Pb/35°C Pb/washed Fe/20°C Fe/35°C Fe/washed Zn/20°C Zn/35°C Zn/washed Cr/20°C Cr/35°C Cr/washed

Xm 0.53 0.53 0.19 1.69 1.54 0.20 0.41 0.41 0.15 0.43 0.43 0.16

b

the

various

Freundlich n

6.45 0.98 6.45 0.98 29.34 1.87 1.76 0.85 1.18 1.28 1 3 . 1 1 2.00 22.20 1.40 22.20 1.40 18.33 1.89 10.22 1.04 10.22 1.04 16.8 11.11

K

Nelson KA

4.02 4.02 0.61 5.40 1.01 0.30 2.14 2.14 0.40 7.40 7.40 0.16

3.57 3.57 1.92 4.23 1.05 1.40 4.64 4.64 0.57 9.95 9.95 --

• EDTA treated;

properties which may be considered as being of significance to the binding of metals include pH, temperature, the oxidation-reduction potential and the presence of complexing agents. From the results of the present study it would appear that the complexation of chromium, lead and zinc does not depend upon the temperature. However, the iron equilibration (Fig. 2) is affected by temperature. Kinetically controlled processes or reactions having a high activation energy would behave in this way. In principle there are two different ways for metal to be concentrated by sludge; by chemisorption through an ion exchange process at the sludge surface or as an inorganic precipitate (Gould and Genetelli, 1984). Except for few salts of calcium and magnesium (e.g. CaCO3), the solubility of an inorganic precipitate increases with temperature. Therefore, any inorganic precipitation of zinc, chromium or lead in the present study is not a deciding factor in the equilibration between liquid and solid phases. Ion exchange reactions are fast reactions (Cheng et aL, 1975) and as such will not be affected by temperature. It is suggested therefore, that the binding of zinc, chromium and lead to digested sludge is a sorption process involving cation exchange. The behaviour of iron (ferrous) is different from that of the other metals. The fact that the equilibration has not been carried out under strictly anaerobic conditions means that the oxidation of ferrous to ferric could be occurring (E0(FclI)_(F¢III) = 0.77) in which case the ferric iron would be readily

Table 3. The effect of EDTA washing in relation to the production of soluble organic matter TOC concentration (mgl ~) EDTA concentration (M 1-])

EDTA solution

Sludge liquors

Extracted material

4 x 10 -3 1.6 x 10 -3 8 x 10 -4 4 x 10 -4 2 x 10 -4 1 x 10 4 Distilled water

480 192 96 48 24 12 0

725 330 225 190 175 160 80

165 58 49 62 71 68 --

1486

K . R . K . ALIBHA[et al.

precipitated as a hydroxide (K,p = 10 ~"). The rate and extent of this conversion are dependent on temperature and the amount of oxygen available. It is suggested therefore that, at 35 C, this oxidationprecipitation of iron competes with the sorption processes making iron behave differently from lead, chromium and zinc. The treatment of sludge with EDTA can (i) extract metals already present in the sludge, thus generating more sites for adsorption; (ii) extract the extracellular polymers and perhaps reduce the metal binding capacity; (iii) change the nature of binding sites; and (iv) render the sludge inactive. Inactivation does not affect the binding capacity (Nelson et al., 1981; Cheng et al., 1975). Nelson et al. (1981) studied both the active and passive uptake of metals by using fresh and autoclaved activated sludge. The adsorption isotherms resulting from this study indicated that there was no real difference in the nature of the binding to the two types of sludge, although the data with the passive sludge were always more scattered and less reproducible. It was considered that the release of soluble organics from the cells together with cell lysis probably contributed to these erratic observations. However, no consistent pattern was evident either for an increase or a decrease in uptake by the live sludge in relation to the dead one. Cheng et al. (1975) also compared the efficiency of the metal uptake by viable and non-active sludge. The nonactive sludge was produced either by sterilization in a steam autoclave for 10rain at 121 'C and 15psig (l.05kgcm 2) or by blending for 10min in a high speed mixer. This work showed that the metal uptake by the original, untreated sludge was greater than the processed sludge. This was possibly due to the release of soluble organic matter by the blending process (Singh and Patterson, 1972). The EDTA does not appear to extract metals from the sludge. The metal concentration in the sludge, as determined by wet ashing, is quite low and the EDTA extract does not show any significant concentration of the metal. However, the EDTA treatment does reduce the alkalinity of the sludge from 3000 to 400 mg 1-L (Table 1) although the pH is not changed. This reduction in alkalinity may be due to the removal of anionic organic ligands such as phosphoryl, carboxyl, sulphydryl or hydroxyl groups of the membrane protein and the lipids of cell wall structural components (Nelson et al.. 1981t. This ~s evidenced by the fact that EDTA does extract organic matter from the sludge (see Table 3). However. the EDTA extracts do not contain inorgamc carbon (although the original sludge liquors dol and secondary extracts obtained by treating the washed sludge solids with distilled water also contained no inorgamc carbon. This means that carbonates/bicarbonates present in the untreated sludge must be precipitated as the organic ligands are released. For these reasons, EDTA-treated sludges would be expected to have a lower initial metal uptake (i.e. controlled by cationic

exchange). The results obtained m this study (Fig. 5) show that such a reduction does in fact occur which is consistent with previous work (Cheng et , / i975; Hayes and Theis, 1976). The constants of equilibration, obtained from the Freundlich and Langmuir isotherms and from Nelson's model representing adsorption of metal ions by sludge surface (Nelson et al., 1981; Nelson and Fristoe, 1983) have been given in Table 2. According to Nelson, at equilibrium: KA

[MS] [M]{S}

(1)

where M = free metal species S = unoccupied surface site MS = metal sludge surface complex KA = conditional adsorption constants (lg -t) [ ]denotes concentration in moll-~ and { denotes concentration in g 1-~.

}

Also ST = {S} + [MS]/X~,

(2)

where Sr = total sludge mass (g 1-~) Xm = number of sites per unit mass of sludge (mol g t). Rearranging equations (1) and (2) gives; [MS] ST

Xm[M] -

Xm/K A +

[M]"

(3)

Equation (3) is similar in style to the Langmuir equation QbC~ q~

1 - bC

(4)

where q, --- MS/ST; KA = Qb; X,, = Q and in the linear part of the isotherm, KA = qe/C,.. The data produced by analysing the results in terms of standard isotherms are shown in Table 2. On the basis of these equilibrium constants; in particular b and n which offer some quantification of the intensity of attachment, it is possible to derive a s e t of binding series. These show (Table 4) that at 20 and at 35°C (if iron is ignored because of its potential for oxidation and precipitation) the series are the Table4. A comparisonof the bindingseries derived from the Langmuirand Freundlich constants System Bindingseries 20°C b n

35~'Cb n Washedb n

Z n > C r > Pb > Fe

Zn> Cr > Pb > Fe Zn > Cr > Pb Zn > Cr > Pb Pb > Zn > Cr > Fe Cr > Fe > Zn>Pb

Heavy metal binding to digested sludge 1.4 -

1.2

Y, 1.0

08

0.6

I

I

I

I

-I ~ - "

2

4

6

8

10

I 12

13

Fig. 6. The relationship between the Freundlich index (n) for EDTA treated digested sludge and the ionic radius of the adsorbed metal ions. same whether derived from b or n. A comparison of the current data with that reported by Gould and Genetelli (1978a) can only be made with zinc (the only metal common to both studies). Such a comparison (Fig. 5) shows that, at a pH of 7, the Freundlich isotherms are almost identical. This means that, for digested sludges, the mechanism of adsorption could be independent of the source of the sludge. This suggestion is not too outrageous since the environment within a digester tends to be controlled more rigorously than that within an activated sludge aeration tank, and, since the environment in a digester will be very similar to that in any other digester, the microbial ecology will be less variable. The data in Table 4 which relate to the washed sludge show a different pattern from that derived for the unwashed sludge and the sequence in the binding series appears to depend on whether the series is derived from b or n. In addition, unlike the data for the untreated sludge, the n values for the washed sludge follow a similar pattern in relation to the ionic radii of the metals involved (Fig. 6) to that reported previously for activated sludge (Forster, 1983). The reasons for these differences are unclear. However, it is reasonable to surmise that the different behaviour of the untreated sludge is the result of the soluble and semi-soluble polymers which are associated with the sludge solids. Increases in the amount of this type of material have been shown to affect binding (e.g. Singh and Patterson, 1972). The EDTA treatment will reduce their concentrations thus producing the differences in binding characteristics. The mechanism of heavy metal removal under anaerobic conditions can involve four distinct processes; adsorption, incorporation into cells, precipitation and the subsequent entrapment of the precipitate in the biofloc and chelation of soluble metal species by organic and inorganic ligands (Cheng et al., 1975). The possible interaction of the metal with, for example, the hydrogen sulphide/sulphide system and with the carbon dioxide/carbonate system can precipitate the metal. Thus in the present set of experiments, the precipitation of zinc as zinc sulphide (Ksp= 1.2 × 1 0 - 2 3 ) ; lead as lead sulphide (Ksp= 3.4× 10-28); chromium as chromium hydroxide

1487

(Ksp = 1 x 10 -36) and the oxidation of ferrous to ferric followed by the precipitation of ferric hydroxide (Ksp = 1.1 x 10 36) would always compete with the metal/surface interactions. Chemisorption through ion exchange is a function of the ionic valency and the ionic radius of the metal. Therefore, on the basis of valency, the chromium(III) should be adsorbed most strongly whereas, based on the ionic size, the adsorption of lead should be the most intense. However, these simplistic extremes will be affected by the nature of the ionic species being adsorbed. The metal ion (M "÷) may exist as (MOH) "~+ and/or (MHCO3) "2÷. Therefore, the charge, size and nature will decide the overall ionic exchange process. The adsorption constants, calculated as the slope of the adsorption isotherm at low metal concentrations, measure the combined effect of the precipitation and adsorption (Nelson et al., 1981). On the basis of the KA values in Table 2, a set of affinity series can be proposed; Cr>Zn>Fe>Pb at 20°C Cr > Zn > Pb > (Fe) at 35°C Pb > Fe > Zn for washed sludge. The soluble metal concentration which is in equilibrium with the sludge solids is quite high (0.3-1.0mmoll -~) and, since none of the concentrates contained any inorganic carbon, it was considered that carbonate precipitation was not occurring. Hydroxide precipitation usually takes place at higher concentrations therefore, since the/CA values were calculated from the low-concentration part of the isotherm, it is suggested that these affinity series are a realistic representation of surface/metal interactions. They do however, present a very different sequence from those based on the simple Langmuir and Freundlich analyses (Table 4). It is suggested that the reason for this is that the two sets of series are depicting two quite distinct aspects of the surface/metal interactions. The one based on b and n quantifies the relative strength of the binding interactions whereas the series based on the Nelson constant (KA) depends on the position of the equilibrium between the metal ions and the receptor ligand. As such, the relative position of the metals in this latter series may depend on the number of sites available to a metal as well as the binding intensity. If the binding series and the affinity series are in fact different it ought to be possible to define a third series, based on capacity. An examination of the values of X,, and K in Table 2 indicates that a capacity series based on Xm is constant for all three Table 5. A c o m p a r i s o n of capacity series derived f r o m L a n g m u i r constant (Xm) System 20°C 35°C Washed

Fe > Pb > C r > Zn Fe > Pb > C r > Z n Fe > Pb > C r > Zn

1488

K . R . K . ALmHA1et at.

systems (i.e. both temperatures and the washed sludge) whereas one based on K is not (Table 5). Also, there is no consistent pattern relating either of these capacity series to any of the series discussed previously. The natural extension of these facts is to question how metal/surface interactions should be quantified and to ask how (and if) the various series reported previously can be related to one another. The answer to these questions is beyond the scope of this paper and further work is required to provide one.

CONCLUSIONS Adsorption isotherms have been used to examine the interactions between anaerobically digested sludge, both as sampled and after washing with EDTA, and four heavy metal ions [zinc, lead, iron (II) and chromium (III)]. The adsorption was measured at 20 and 35'~C and the results expressed as Freundlich and Langmuir isotherms. These were then used to calculate not only the constants associated with each isotherm type but also the affinity constants as reported by Nelson et al. (1981). The various constants were then used to derive attachment series for the metals under consideration. This showed that although the series based on the Langmuir--b and the Freundlich--n were similar, the other series (based on the Langmuir--X,, and the Nelson--KA) were different. On this basis, therefore, it is suggested that the various series are depicting different and distinct aspects of the interactions between the sludge surfaces and the metal ions. Thus b and n assess the strength of the binding; K^ provides a measure of the overall affinity (i.e: a combination of the number of sites available and the binding intensity) whilst X,, measures the capacity of the surface. The question that is not, and cannot, at this stage be answered is how metal/surface interactions ought to be described. Comparisons between the data for the untreated and washed sludges indicate that the reduction in the initial metal uptake caused by the EDTA washing is likely to be caused by a removal of soluble/

semi-soluble polymers associated with the sludge solids. However, with the exception of iron(lI)~ the binding of which appears to involve oxidation/ precipitation, the general sorption mechanism is suggested as being one involving cation exchange to surface ligands. REFERENCES

Cheng M. H., Patterson J. W. and Minear R. A. (1975) Heavy metal uptake by activated sludge. J. War Pollut. Control Fed. 45, 362. Forster C. F. (1983) Heavy metals and activated sludge surfaces. Envir. Technol. Lett. 4, 417. Gould M. S. and Genetelli E J. (1978a) Heavy metal complexation behaviour in anaerobically digested sludges. Water Res. 12, 505 Gould M. S. and Genetelli E. J. (1978b) The effect of methylation and hydrogen ion concentration on heavy metal binding by anaerobically digested sludges. Water Res. 12, 889. Gould M. S. and Genetelli E. J. [1984) Effect of competition on heavy metal binding by anaerobically digested sludges. Water Res. 18, 123. Hayes T. D. and Theis T. L. (1978) The distribution of heavy metals in anaerobic digestion. J. War Pollut. Control Fed. 50, 61. Lake D. L., Kirk P. W. W. and Lester J. N. t1984) Fractionation, characterization and speciation of heavy metals in sewage sludge and sludge amended soils: a review. J. envir. Qual. 13, 175. Lewis T. E. and Broadbent F. E. (1961) Soil organic matter-metal complexes--IV. Nature and properties of exchange sites. Soil Sci. 91, 383. Nelson P. O. and Fristoe B. R. (1983) Equilibrium modelling of heavy metals in activated sludge. Water Res. 17, 771. Nelson P. O., Chung A. K. and Hudson M. C (1981) Factors affecting the fate of heavy metals in the activated sludge process. J. Wat. Pollut. Control Fed. 53, 1323. Rivera A. L. (1983) Heavy metal removal in a packed-bed, anaerobic upflow (ANFLOW) bioreactor. 1. War Pollut. Control Fed. 55, 1450. Schnitzer M. and Skinner S. I. M. t1965)Organo-metallie interactions in soils---4.Carboxyl and hydroxyl groups in organic matter and metal retention. Soil Sei. 99, 278. Singh T. and Patterson J. W. (1972) Improvement of the aerobic sludge digestion process efficiency. Paper presented at 45th Annual Conference. Water Pollution Control Federation, Atlanata, Ga. Tan K. H., King L. D. and Morris H. D. (1971) Complex reactions of zinc with organic matter extracted from sewage sludge. Soil Sci. Sot' Am. Proc 35, 748