Solubility and uptake of heavy metals from a Swedish soil

Solubility and uptake of heavy metals from a Swedish soil

Geoderma, 19 (1977) 123--129 123 © Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands SOLUBILITY AND UPTAKE OF HE...

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Geoderma, 19 (1977) 123--129 123 © Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands



Department of Soil Science, Swedish University of Agricultural Sciences, Uppsala (Sweden) (Received November 1, 1976; accepted June 8, 1977)


Wiklander, L. and Vahtras, K., 1977. Solubility and uptake of heavy metals from a Swedish soil. Geoderma, 19: 123--129. Samples of the surface layer (Ap) and of grass, collected from: (1) grass ley fertilized in the normal way; (2) permanent pasture fertilized in the normal way; and (3) permanent pasture treated with large amounts of sewage sludge five years earlier, were analysed for Mn, Zn, Cu, Cr, Co, Ni, Pb, and Cd. The soil samples were extracted with: (1) distilled water saturated with CO2; (2) 1 M neutral a m m o n i u m acetate; (3) a m m o n i u m acetate + acetic acid, pH 4.75; and (4) 2 M nitric acid on a waterbath. The efficiency of these extractants differed greatly and, in relative values, was: 1 for H~O + CO~, 3.4 for NH4OAc, 20.7 for NH4OAc + HOAc, and 343 for 2 M HNO 3. The dissolving effects of the extractants differed markedly with the kind of element. Grass from the field treated with sewage sludge showed much higher contents of Mn and Zn and somewhat higher contents of Cu and Pb than grass from the untreated field. The levels of Cr, Co, Ni, and Cd were practically uninfluenced by the treatment. Grass from a field close to a highway accumulated large amounts of air-borne Pb and Cd during the sumnler. It is concluded that the total contents of heavy metals in soils have only limited importance for the uptake by plants. Weak extractants therefore give better information about the plant-available amounts in soils.


The use of sewage sludge from communities in agriculture as an organic manure and for soil a m e n d m e n t has positive as well as negative effects. Sludge has been found to have a positive effect on yields. On the other hand, the contents of certain toxic heavy metals are often higher than in ordinary manure (Andersson, 1977). This has led to restrictions for its use in agriculture in Sweden, i.e., a recommendation n o t to exceed 1,000 kg dry matter per ha per year as a mean. In connection with a study of the nitrogen c o n t e n t in ground water and drainage water at Alnarp Agricultural Institute, near MaimS, southern

12,4 Sweden, during 1972----1973 (Wiklander and Vahtras, 1975) soil samples were taken from a field (No. P--6) treated with large amounts of sewage sludge in 1970 and from an adjacent field (No. P--8) w i t h o u t sludge treatment. The samples were analysed for heavy metals. It was found that the heavy metals dissolved by 2 M HNO3 on a waterbath were m a n y times higher (Zn 9, Cu 14, Cr 6, Pb 5 and Cd 7 times) in samples from the field previously treated with sludge than in samples from the adjacent field {cultivated) where sludge had never been used (Table I). In order to obtain more information about the significance of heavy metals in soils the investigation was re-opened in 1975, this time broadened with a study of the solubility in various extractants and the uptake by plants under field c o n d i t i o n s TABLE I Contents of heavy metals (ug/g of soil) in soil samples from pasture treated with large amounts of sewage sludge (P~-6) and from an adjacent field without sludge (P--8), and mean values of 363 cultivated soils from different parts of Sweden extracted with 2 M HNO~

(Andersson, 1975) Field









P--6 (with sludge) P--8 (without sludge) 363 cultivated soils

130 174

334 37

86 6.1

24.5 4.1

5.6 4.9

12.4 5.6

25.4 5.0

1.39 0.21









METHODS AND EXPERIMENTAL PLAN The surface layer, (Ap horizon), and vegetation were sampled on three fields (eutric cambisols) the sampling places being referred to as Nos. P--l, P--5, and P--6. The vegetation on the three fields consisted of grass, but of somewhat variable botanical composition. P--5 and P--6 were permanent pastures, and P--1 was second-year grass for hay. Each soil sample was composed of several small samples taken at random by means of an ordinary soil borer to a depth of 20 cm and mixed. After air drying, the soil samples were passed through a 2 mm sieve. The grass sampled had a height of 15--25 cm and was cut just above t h e ground. F o u r samplings were made: June 16, July 4, August 20, and September 30. The samples were dried at 50°C and ground. The solubilities of the heavy metals in the softs were determined by extraction with four different solutions: distilled water saturated with carbon dioxide; 1 M a m m o n i u m acetate (pH 7.0); 0.5 M a m m o n i u m acetate + 0.5 M acetic acid {pH 4.75); and 2 M nitric acid (on a waterbath). The soil: solution ratio was 1:10 and the extraction time 2 hours in all cases. An end-


over-end rotator was used for shaking. The chemicals were of analytical grade, Merck's Suprapur, and the analysis, in duplicate, was carried o u t by means of an atomic absorption s p e c t r o p h o t o m e t e r with a deuterium background corrector and graphite furnace. The vegetation samples were dryashed according to the procedure of Andersson (1976) and the ashes dissolved in nitric acid. Sampling point P--1 is situated 2,000 metres from points P--5 and P--6. P--1 was chosen as a reference field with a vegetation of a b o u t the same character (grass) as on the other two fields, both of which had been in use as pasture for several years. P--5 is located very near a highway and P--6 is a b o u t 250 metres from this highway. RESULTS AND DISCUSSION

Solubility o f heavy elements in the soils The solubility data are given in Table II as mean values of four samplings during June-September. The solubility increases in the same order as the strength of the extractants: H20 + CO2 < NH4OAc, pH 7.0 < NH4OAc + HOAc, pH 4.75 < 2 M HNO3, b u t the relative solubilities, expressed as percentages of the total extractable amounts, are n o t the same for the various elements (Table III). As an average of all the elements analysed the solubilities in the four extractants form the following proportions: 1 : 3.4 : 20.7 : 343. Among the extractants used the H20 + CO2 is the weakest and most alike the soil solution that is often CO2-saturated. The neutral and acidic acetates have concentrations far above those of normal soil solutions and replace the major part of the metals in exchangeable form. NH4OAc (pH 4.75) also dissolves easily soluble oxides and complexes, especially those of Mn, Zn and Cd, which may explain the high solubility efficiency compared with NH4OAc (pH 7) and HNO3 (Table III). The major part of Cd may be held by clay and organic matter as exchangeable Cd 2÷, CdOH ÷ or CdC1÷. The bonding energy is high, resulting in a relatively slight uptake b y plants. Cd 2÷ is t o o large (radius 1.03 £ according to Goldschmidt) to become fixed in tetrahedral and octahedral positions b y clay minerals. 2 M HNO3 is the most efficient extractant, bringing into solution n o t only exchangeable ions b u t also oxide- and complex-bound forms and to some extent also ion constituents of the mineral structure. This extractant dissolves a b o u t 80% of the total content. The P--1 soil (cultivated sandy loam) has higher contents of heavy metals soluble in HNO3, with the exception of Pb and Cd, than the P--5 soil (pasture on loamy sand), neither of them ever having been treated with sewage sludge. The difference is likely to depend on the higher clay c o n t e n t in P--1


T A B L E II S o l u b i l i t y in f o u r e x t r a c t a n t s o f h e a v y m e t a l s in soil a n d the c o n t e n t s in v e g e t a t i o n (grass) at A l n a r p . M e a n values o f f o u r s e p a r a t e v e g e t a t i o n samplings in J u n e - - S e p t e m b e r 1975 (ug/g ! P--6 t r e a t e d w i t h large a m o u n t s o f sewage sludge in 1 9 7 0 , P--1 a n d P--5 u n t r e a t e d Fields









<0.02 <0.02 <0.02

<0.05 <0.06 0.20

<0.04 <0.04 <0.04

<0.01 <0.01 <0.01

SoiI H~O + C O P--1 P -5 P--6


0.026 0.028 0.102

1 M NH4OAc, P--1 P--5 P--6

pH 7.0 4.50 3.84 4.33

0.022 0.034 0.087

2.81 4.63 30.80

NH4OAc + H O A c , p H 4.75 P--1 53.6 17.4 P--5 33.1 49.3 P--6 37.7 237.2

2 M HNOs, 100°C P--1 169 P--5 89 P--6 197

76 55 1084

0.193 0.245 1.490

0.014 0.016 0.080

0.21 0.28 3.75

<0.01 <0.01 <0.05

<0.05 <0.05 <0.05

0.10 0.13 0.44

0.06 0.37 0.36

0.07 0.12 0.37

0.53 0.60 8.75

0.57 0.56 1.30

0.33 0.18 0.35

0.33 0.28 2.43

4.13 4.62 5.90

0.10 0.42 1.65

16.3 11.2 228

11.6 3.8 71.9

4.7 2.2 4.5

9.9 5.8 24.4

16.8 27.8 98.6

0.46 0.69 2.47

1.42 14.35 3.07

0.13 0.16 0.16

Vegetation P--1 P--5 P--6

21.8 22.1 82.1

36.5 35.5 82.1

8.1 7.0 10.4

0.52 0.64 0.56

<0.25 <0.25 <0.25

1.18 1.19 1.31

T A B L E III S o l u b i l i t y in d i f f e r e n t e x t r a c t a n t s o f h e a v y m e t a l s in soil t r e a t e d w i t h sewage sludge (P--6). Relative values, NH4OAc (pH 4 . 7 5 ) = 1 Elements

Mn Zn Cd Cu Cr Co Ni Pb Rel. p r o p o r t .

H 2 0 + COs

< < < <

0.0027 0.0004 0.006 0.17 0.06 0.06 0.08 0.007 1

NH4OAe pH 7

NH 4 0 A e p H 4.75

HNO 3 2M

0.11 0.13 0.22 0.43 < 0.04 < 0.14 0.18 0.06 3.4

1 1 1 1 1 1 1 1 20.7

5.2 4.6 1.5 26.1 55.3 12.9 10.0 16.7 343


and a certain contribution via fertilizers regularly applied to this soil for many years. As expected, application of sewage sludge to the P--6 soil resulted in all metals except Co being higher in P--6 than in the P--1 and P--5 soils; Zn, Cu, Cr, Pb, and Cd being more than five times higher.

Contents of heavy metals in vegetation The contents of all metals except Pb in samples from P--1 and P--5 are a b o u t the same (Table II). The corresponding values in vegetation from the sewage-treated P--6 are also fairly similar. Only Mn and Zn are much higher than in P--1 and P--5, whereas Cu and Pb are only slightly higher. There are no values available which could be taken as normal contents in grass. However, we can consider these values as n o t being extremely high. The most hazardous metal, Cd, is only slightly higher in samples from P--5 and P--6 than in samples from P--l, although both the HNO3 extractable and acetate extractable Cd are much higher in the P--6 sample than in the o t h e r two. It is striking that the Pb content is much higher in vegetation from P--5 (14.4 tLg/g) than in samples from P--6 (3.1 ,~g/g) despite the fact that the P--6 field had been treated with very large amounts of sludge in 1970 and the soil has a high level of extractable Pb. The reason for this difference must be found elsewhere. It has been shown that lead in the soil is relatively inert and is n o t readily taken up by plants (MacLean et al., 1969}. Also in solution lead is taken up b y plants only to a slight extent. Thus, lead was n o t found in noticeable amounts in leaves b y Carlson et al. (1975) in experiments with culture solutions with sunflower and maize, whereas the cadmium c o n t e n t in leaves increased with increasing Cd c o n t e n t in culture solution. The elevated lead levels in plants growing near highways are primarily due to direct pollution by airborne lead from m o t o r vehicles (Wiklander, 1970, 1971; M o t t o et al., 1970). The very high Pb c o n t e n t in vegetation from P--5 in September (35 pg/g) (Table IV, Fig.l) must therefore originate from the highway, from which the distance to the sampling place P--5 is only a few metres. Vegetation from the sewage-treated P--6, where the soil contains a b o u t four times more HNO3soluble Pb than in P--5, shows one fifth of the Pb c o n t e n t of vegetation from the untreated P--5. P--6 is located a b o u t 250 m from the highway and therefore outside the direct pollution distance of the traffic (Wiklander, 1970, 1971). Close to the highway (P--5) the contents of Pb, b u t also of Cd, have increased markedly during the summer, for Pb from 2.75 to 35 pg/g and for Cd from 0.075 to 0.35 ~g/g (Fig.l). A clear b u t lesser increase is shown at P--6. Cd-contamination along roads is attributed to abrasion of tyre rubber (Lagerwerff and Specht, 1970).


TABLE IV Contents of Pb and Cd in grass Sampling date

June July August Sept. Mean

Pb (/~g/g of d.w.)

Cd (ug/g of d.w.)


16 4 20 30







1.25 2.00 1.00 -1..42

2.75 7.75 11.90 35.00 14.35

1.30 1.25 2.75 7.00 3.07

0.035 0.190 0.173 -0.133

0.075 0.085 0.113 0.350 0.156

0.165 (0.035) 0.078 0.355 0.158

#gig Pb Cd 40 "0.4 Pb

~ - - -

Cd ....... 30




10 f0.1 ~ + i + Pb


O June

,C ~ July


/to i



September 1975

Fig. 1. Contents o f lead in grass from three different places (P--l, P--5, and P-'6), and o f Cd from P--5 during the summer. P--1 represents cultivated soil 2,000 m from highway, P--6 pasture 250 m from highway (soil rich in Pb due to addition o f sewage sludge) and P--5 pasture close to highway. (In ug/g dry matter.)

Comparison between the solubilities of heavy metals in the four extractants and the uptake by vegetation shows that only a slight fraction of the metals is plant-available. The strong extractants bring into solution a large part of the inert forms of the elements. Therefore weak extract,ants give a more reliable estimation of the plant, available amounts of heavy metals in soils. However, it must be kept in mind that the solubility of most heavy metals increases with decrease of soft pH. Also the redox conditions can have a strong bearing on the solubility of certain elements. The concentration of C1- may also have some effect.


REFERENCES Andersson, A., 1975, Vissa tungmetallers geokemi. Milj~forskning 1973-1976. Statens Naturv~rdsverk, Publikationer 1975, 3: 25--26. Andersson, A., 1976. On the determination of some heavy metals in organic material. Swedish J. agric. Res., 6: 145--150. Andersson, A., 1977. Some aspects on the significance of heavy metals in sewage sludge and related products used as fertilizers. Swedish J. agric. Res., 7 (In press). Carlson, R.W., Bazzaz, F.A. and Rolfe, G.L., 1975. The effect of heavy metals on plants. II. Environ. Res., 10: 113--120. Lagerwerff, J.V. and Specht, A.W., 1970. Contamination of roadside and vegetation with cadmium, nickel, lead and zinc. Environ. Sci. Technol., 4: 583--586. MacLean, A.J., Halstead, R.L. and Finn, B.J., 1969. Extractability of added lead in soil and its concentration in plants. Can. J. Soil Sci., 49: 327--334. Motto, J.L., Daines, R.H., Chilko, D.M. and Motto, C.K., 1970. Lead in soils and plants: Its relationships to traffic volume and proximity to highways. Environ. Sci. Technol., 4: 231--237. Mills, J.G. and Zwarich, M.A., 1975. Heavy metal content of agricultural soils in Manitoba. Can. J. Soil Sci., 55: 295--300. Wiklander, L., 1970. Bly i mark och v~ixter. I. Inverkan av motortrafik. (Lead in soil and plants. I. Influence of m o t o r vehicles.)Grundf'6rb~ittring, 23: 163--169. Wiklander, L., 1971. Bly i mark och v~ixter. II. Vid motorv~ig i Sk~ne. (Lead in soil and plants. II. At a highway in Scania.) Grundf~rb~ttring, 24 : 65--67. Wiklander, L. and Vahtras, K., 1975. Kv~ve i markvatten och tungmetaUer i r~tslamgSdslad jord p~ Alnarp. (Nitrogen in soil water and heavy metals in soil with applied sewage sludge at Alnarp.) Landbruksh~gskolans medd., Ser. A, Nr 241.