HEAVY METAL BINDING BY ORGANIC SUBSTANCE IN SEWAGE SLUDGE OF VARIOUS ORIGIN

Krzysztof Gondek, Michał Kopeć
Department of Agricultural Chemistry, Agricultural University of Cracow, Poland

ABSTRACT

Due to an important role of organic substance, particularly humus acids in forming chelate complexes with heavy metals in soil, research was carried out aimed at determining quantitative and qualitative composition of organic substance in sewage sludge and the amount of heavy metals bound by it. Organic substance in sewage sludge of municipal origin retained more cadmium and zinc, whereas in tannery sludge more chromium and lead. Comparable amounts of copper and nickel bound by organic substance were detected in all researched sewage. Total contents of trace metals in the sludge is not a sufficient criterion for the assessment of its usability for agricultural use, because the occurrence of considerable amounts of some elements in organic compounds (recognized as potential source of trace metals) may pose a hazard of their excessive uptake by plants. Contents of carbon and fractional composition of organic substance in the investigated sewage sludge point to them as a possible source of humus forming substances once they meet the microbiological and chemical requirements.

Key words: sewage sludge, heavy metals, organic substances.

INTRODUCTION

According to regulations and reason wastes should be managed in the way to ensure protection of life and health of people and be safe for natural environment. Various kinds of sewage sludge are a problematic group of organic wastes. Their amounts are considerable: 359 thousand tonnes of municipal wastes and 703 thousand tonnes dry mass of industrial wastes in 2000 and further symmetrical increase in their quantity are envisaged posing even more serious problems with their management [1].

Sewage sludge provides a huge potential of nutrients for plants and organic substance, which classifies it for fertilizer use [7]. Classification of sewage sludge for biological use is legally permissible [13] and bases primarily on the assessment of heavy metal contents and its sanitary properties.

It seems that beside total heavy metal contents, it is important to know the kinds of heavy metal bonds, which occur in sewage sludge [9]. Water-soluble forms of heavy metals, exchangeable and bound to carbonates are regarded as the most mobile forms i.e. easily passing into the soil solution. Metal fractions strongly bound to constant phase of sewage sludge, mainly to iron oxides and organic substance are also a potential source of these elements for plants, however their release (and uptake by crops) from the sludge is slower.

Because of a significant role of organic substance, particularly humus acids in a formation of chelate complexes with heavy metals in soil, research was carried out aimed to determine quantitative and qualitative composition of organic substance in sewage sludge and amounts of heavy metals bound by it.

MATERIAL AND METHODS

Samples of sewage sludge for the studies were collected to differ with their place of origin and heavy metal contents. The materials were sampled in 2 municipal treatment plants (mechanical and biological) at Niepołomice and Krzeszowice, from 1 municipal and industrial treatment plant (mostly treating sewage mechanically) in Krakow-Płaszow and from 2 industrial sewage treatment plants treating tannery sewage (mechanical and biological) in Kraków-Bieżanów and (chemical) in Radom. A mean sample for chemical analyses was prepared from initial collected samples. The analysed farmyard manure was treated as a reference for comparable features. Prior to sampling all sewage sludge was stabilised for various periods of time and using different technologies.

In the fresh material dry mass was determined after the samples drying at 105°C for 12 hrs. In the dried and ground material pH was measured in water – with a potentiometer, total nitrogen content by Kjeldahl’s method using Kjeltec II Plus apparatus (Tecator), organic carbon concentration after mineralization in potassium di-chromate(VI) by Tiurin’s method. The contents of potassium, phosphorus, calcium, magnesium, sodium and mercury were determined in the sludge after dissolving the annealed samples (450°C for 4 hrs) in nitric acid(V) (1:2). Phosphorus was assayed in a colorimeter by vanadium and molybdenum method using DU 640 Spectrophotometer (Backman) while potassium, sodium and calcium by flame photometry (FES) and magnesium by atomic absorption (ASA) in PU 9100X spectrophotometer (Philips). Mercury was determined by cold vapour using PU 9100X spectrophotometer (Philips) with Unicam VP 90 attachment (Table 1).

Humus compounds were extracted from the sewage sludge by a solution of 0.1 mol · dm^-3 Na₄P₂O₇ + 0.1 mol · dm^-3 NaOH mixture. In the obtained extracts total carbon was determined and carbon of humic acid (by Tiurin’s method), while carbon of fulvic acid was computed from the difference between total amount of organic carbon in the extract and its amount in humic acids [11].

Individual fractions of heavy metals were separated using Tessier et al. method [15] (Table 1). Heavy metal concentrations in the obtained extracts were assayed by ICP-AES method in JY 238 Ultrace apparatus (Jobin Yvon). Total contents of the studied elements were calculated as a sum of fractions.

The results of analyses presented in the research are a mean of 3 parallel replications.

RESULTS

The analysed sewage sludge differed with dry mass contents. The sludge from the municipal treatment plant at Krzeszowice had the biggest quantities (465 g · kg^-1), the least amount of dry mass was found in tannery sludge from Kraków-Bieżanów (239 g · kg ^-1) (Fig. 1). The diversification might have resulted from different period of sewage sludge storage on sludge beds. All studied kinds of sewage sludge contained more dry matter than farmyard manure (213 g · kg^-1).

The reaction of the studied sludge was not greatly diversified and ranged between 6.37 and 7.81 (farmyard manure 6.80) (Fig. 2).

Macroelement and mercury contents in the investigated sludge were diversified and depended on the place of sewage origin (Table 2). The macroelement contents were discussed in detail in another publication [8]. Mercury concentrations in the analysed materials revealed high variability but fell within the range five times lower than permissible for municipal sludge use in agriculture.

Organic carbon concentrations in the studied sludge remained on a similar level, except for the sludge from Krzeszowice where the least amounts were found (Fig. 1).

The content of carbon extracted from sewage sludge with a mixture of sodium pyrophosphate and sodium base ranged from 13.38 to 40.39 g · kg^-1 dry mass. The biggest quantity of this organic fraction were assessed in tannery sludge and farmyard manure (Table 3).

The highest concentrations of humic acid carbon were determined in tannery sludge from Kraków-Bieżanów and Radom and in farmyard manure, respectively: 24.35g; 15.12g and 23.08g · kg^-1 dry mass. Municipal sludge contained considerably less carbon of humic acids (5.97g – 7.72 g · kg^-1 dry mass) (Table 3).

The contents of fulvic acid carbon were more diversified and the biggest amounts of this fraction of organic substance were detected in the municipal sludge from Niepołomice (18.19 g · kg^-1 dry mass) and the sludge from Krzeszowice had the least of fulvic acid carbon (7.41 g · kg^-1 dry mass) and farmyard manure (7.27g · kg^-1) (Table 3).

The value of humic acids to fulvic acids ratio (CKh : CKf) was the highest in farmyard manure – 3.17 (Fig. 2). Tannery sludge were characterised by a higher value of this parameter (on an average by 1.25) than municipal sludge (on an average by 0.63).

The highest value of non-hydrolysing carbon was assessed in farmyard manure (249.41 g · kg^-1 dry mass). The studied kinds of sludge contained diversified quantities of this fraction, and at the same time tannery sludge revealed lesser variability. The indices are considerably bigger for the soil.

Total chromium concentrations in sewage sludge were diversified and depended on the origin of sludge (Table 4). The biggest amount of chromium was found in industrial sludge from tannery in Krakow-Bieżanow (4872.10 mg · kg^-1) and Radom (967.89 · kg^-1 sludge dry mass), connected with this element share in the technological process as corroborated by some earlier studies conducted by Filipek-Mazur and Gondek [5]. Big difference in chromium contents among the analysed kinds of tannery sludge from Kraków-Bieżanów and Radom resulted from different sewage treatment technologies. Chromium content in the other kinds of sewage sludge was bigger than found in the farmyard manure (6.44 mg · kg^-1dry mass), but it did not exceed 500 mg Cr · kg^-1, which is the value permissible for municipal sludge for non-industrial use [13]. On the basis of obtained results of sewage sludge heavy metal concentrations it may be said that apart from excessive chromium content in tannery sludge (from Kraków-Bieżanow and Radom), which exclude these kinds of sludge from agricultural use, the contents of the other heavy metals did not raise objections as to the sludge biological use [13]. Chromium quantities bound by organic matter in the sewage sludge were varied and ranged from 34% to 81% of the total contents, while the organic matter in farmyard manure bound 37% of the element. The obtained results allow for the conclusion that in organic fraction more chromium was found in the sewage sludge of industrial origin (tannery sludge).

The contents of total zinc forms in the studied sewage sludge ranged widely from 171.76 mg – 3197.88 · kg^-1 dry mass) (Table 5). Higher concentrations of this element were assayed in municipal sludge, where they exceeded 1500 mg, whereas in the industrial sludge it did not exceed 200 mg · kg^-1dry mass. The value of total forms of zinc determined in the farmyard manure was 819.84 mg Zn · kg^-1 dry mass. Concerning agricultural utilisation of the studied sewage sludge, the threshold content of zinc (2500 mg · kg^-1) was exceeded in the sludge from Krakow-Płaszow (3197.88 mg · kg^-1 dry mass). The biggest amounts of zinc bound to organic substance were detected in municipal sludge from Niepołomice (51% of total contents) which corresponded to 1161.07 mg Zn · kg^-1dry mass. A smaller share of zinc bound with organic substance was found in the sludge from Krakow-Płaszów at a completely higher value 1319.47 mg · kg^-1 dry mass. Organic substance in tannery sludge bound the least of zinc, on an average 25.5%.

Total contents of nickel in the studied kinds of sewage sludge did not pose any environmental hazard [13]. The biggest quantities of this element were found in municipal sewage sludge (Table 6). Smaller amounts were assessed in tannery sludge and farmyard manure. Organic substance in the farmyard manure and sewage sludge from Niepołomice retained the most of nickel, respectively 61 and 43% of the total contents. On an average organic substance in the tannery sludge bound 33% of this element, and the least of nickel in this fraction, on an average 25.5%, was assayed in the sludge from Krzeszowice and Kraków-Płaszów.

Cadmium is one of the metals more dangerous for the natural environment. Its total contents in the sludge from Kraków-Płaszów, which had the most of Cd, was over twice lower than permissible contents of this element in sludge destined for agricultural use [13] (Table 7). Cadmium reveals high affinity for forming complexes with organic substance and in the presented research the amount of cadmium bound to the sludge organic substance ranged from 25 to 59% of the total contents. Smaller amounts of Cd stored in organic compounds were determined in the tannery sludge. For comparison, the farmyard manure organic substance bound only 12% of the total cadmium contents, which corresponded to 0.15 mg · kg^-1 dry mass.

The contents of copper total forms in the studied sewage sludge were very diversified and ranged from 30.6 mg and 480.7 mg · kg^-1 dry mass, but farmyard manure had the biggest concentrations of this metal – 703.0 mg · kg^-1 dry matter (Table 8). So high concentration of copper in the farmyard manure was due to its origin and most probably to the way of nutrition of pigs. On the farm, which supplied the manure, pigs were intensively fed concentrates and additionally mineral and vitamin concentrate to meet the animal nutritional needs. The assayed copper contents were lower than considered as permissible for sewage sludge for agricultural use [13]. The studies demonstrated that organic substance of the sewage sludge was the main copper binding element. The amount of copper bound by the organic substance constituted on an average 84% of the total contents with slight fluctuations (87% in the farmyard manure).

Total lead concentrations in the tannery sludge were 2.91–14.88 mg · kg^-1 dry mass (Table 9). The contents were below the value considered permissible for agricultural utilisation of the sewage sludge [13]. The quantities of lead bound by the organic substance were small. Organic substance of the tannery sludge bound between 4 and 14% of lead, whereas the amount of lead bound with the organic fraction did not exceed 6% of total contents.

DISCUSSION

According to Weber [16] organic substance plays an important role among the factors influencing the processes of binding, mobilising and migration of heavy metals in soil. In soil conditions the durability of humus compound – metal complex depends on the soil reaction and kind of ion. It is bigger at higher values of this parameter. The character of organo-mineral compounds, degree of their saturation with metallic ion, the complex adsorption on a mineral soil particle and also biodegradation of the complex organic component determine the mobility of metal ion in soil conditions [4]. Sewage sludge application to the soil involves a danger of soil environment pollution with toxic substances but also supplies considerable amounts of organic matter which may balance the negative effect of these substances.

The discussed kinds of sewage sludge were diversified as to their contents of organic fraction considered as the most active soil proper humus fraction extracted with a mixture of sodium pyrophosphate and sodium base [11]. The biggest amounts of this fraction were separated from the tannery sludge and farmyard manure. Results of studies on the properties of organic matter in wastes to be used as fertilizers obtained by Flis-Bujak et al. [6] corroborate the results obtained in the presented research but in relation to the farmyard manure. The quoted authors [6] extracted definitely smaller amounts of this fraction when treating sewage sludge with a mixture of sodium pyrophosphate and sodium base. According to Flis-Bujak et al. [6] humic acid fraction prevails in the farmyard manure whereas fulvic acids constitute a majority in sewage sludge and municipal wastes. The obtained research results allow for similar conclusions, except one case (tannery sludge from Kraków-Bieżanów) where humic acid concentrations were higher (Ckh:Ckf – 1.52). According to Flis-Bujak et al. [6] the humus forming value depends on total carbon contents and organic compound susceptibility to mineralization and transformation into humus compounds. Contents of carbon and fractional composition of organic substance in the studied sewage sludge indicate that they may be a source of humus forming substances.

According to Kabata-Pendias et al. [10] total trace metal contents is a fast and sufficient test for determining agricultural usability of sewage sludge. However, the rate of trace metal release to the soil solution is most important for the agricultural practice. For an approximate assessment of this process one must determine the forms in which the elements occur in sewage sludge. Sequential chemical extraction procedures are most often used for this purpose. Forms bound to organic substance, which may be a potential source of heavy metals for plants, constitute a fraction of heavy metals released during extraction. Sewage sludge is a considerable part of waste. Its chemical composition changes and thus it is difficult to predict its effect upon heavy metal availability in soil.

The contents of heavy metals bound to organic matter of the studied sewage sludge primarily depended on the sludge origin. In municipal sewage sludge more cadmium and zinc was found in organic compounds. On the other hand, organic substance in tannery sludge fixed more chromium and lead. Regarding zinc, cadmium and chromium the amount of bound element depended on its total contents. Lead was bound by other mechanisms. Similar quantities of copper and nickel were assayed in the organic fraction of all studied kinds of sludge and farmyard manure where very high amount of copper was found. Kabata-Pendias et al. [10] reached similar conclusions. Piotrowska and Dudka [12] point to high diversification in heavy metal distribution in fractions separated from sewage sludge but at the same time they emphasise a most important role of iron and manganese oxides and organic substance as the factors playing the man role in storing individual heavy metals.

Sewage sludge may be also a source of nutrients as demonstrated by numerous studies [14, 17]. Industrial sludge, including tannery sludge arise bigger controversy as to their biological utilisation. The research conducted by Czekała et al. [3] proved that a negative effect of tannery sludge on plant yield resulted from fatty compounds and not chromium contents. It is understandable considering very low solubility of this element both in municipal and tannery sludge and the resulting low possibility of passing into the soil solution [2, 8]. A fairly large load of sodium supplied to the soil with tannery sludge dose may have also a big importance [5].

CONCLUSIONS

1. Total contents of studied trace elements in tannery sludge were diversified and depended on the studied element and the sludge origin.

2. Organic substance of the municipal tannery sludge retained more cadmium and zinc, whereas in the tannery sludge more chromium and lead. Comparable amount of copper and nickel bound by organic substance were detected in all examined kinds of sewage sludge.

3. Total contents of trace elements in the sludge does not provide a sufficient criterion for the assessment of these substances agricultural usability because occurrence of considerable amounts of some elements in organic bonds (recognized also as potential sources of trace metals) may pose a hazard of their excessive uptake by plants.

4. Carbon contents and fractional composition of organic substance of the analysed sewage sludge reveal that they may be a source of humus forming substances.

REFERENCES

1. Baran S., Oleszczuk P., Żukowska G.; 2002. Zasoby i gospodarka odpadami organicznymi w Polsce. [Organic wastes in Poland – supply and management]. Acta Agrophysica; 73: 17-34; [in Polish].

2. Czekała J., Jakubus M., Mocek A., Owczarzak W.; 1996. Frakcje chromu w kompostach z udziałem osadu garbarskiego i różnych dodatków organicznych. [Chromium fractions in composts with contribution of tanning deposits and various organic supplements]. Zesz. Probl. Post. Nauk Roln.; 437: 117-122; [in Polish].

3. Czekała J., Mocek A., Owczarzak W.; 1993. Działanie nawozowe osadu garbarskiego. [Fertilizer activity of tannery sludge]. Zesz. Probl. Post. Nauk Roln.; 409: 113-118; [in Polish].

4. Dziadowiec H.; 1993. Ekologiczna rola próchnicy glebowej. [Ecological role of soil organic matter]. Zesz. Probl. Post. Nauk Roln.; 411: 267-282; [in Polish].

5. Filipek-Mazur B., Gondek K.; 2000. Dynamika składu chemicznego osadów sciekowych z biologiczno-mechanicznej oczyszczalni scieków garbarskich. [Dynamics of chemical composition of sewage sludge from mechanical – and – biological tannery sludge treatment plant]. Zesz. Nauk. AR w Krakowie; 374; Roln.; 37: 17-24; [in Polish].

6. Flis-Bujak M., Baran S., Żukowska G.; 1996. Własciwoœci materii organicznej wybranych odpadów o charakterze nawozowym. [Properties of organic matter from a selected fertilizing waste]. Zesz. Probl. Post. Nauk Roln.; 437: 147-152; [in Polish].

7. Gondek K., Filipek-Mazur B.; 2001. Agricultural usability of sewage sludge and vermicompost of tannery origin. Elect. J. of Pol. Agr. Univ., En. Develop.; 4; Issue 2; www.ejpau.media.pl/series/volume4/issue1/environment/art-01.html

8. Gondek K., Filipek-Mazur B.; 2003. Mobilnosc chromu w osadach sciekowych różnego pochodzenia. [Chromium mobility in sewage sludges of various origin]. Acta Scient. Polon. Ser. Format. Circumiectus; 2(1): 51-60; [in Polish].

9. Jakubas M.; 2003. Changes in zinc combinations in sewage sludge with and without an addition of lime. Elect. J. of Pol. Agr. Univ., Environ. Develop.; 6; Issue 2; www.ejpau.media.pl/series/volume6/issue2/environment/art-06.html

10. Kabata-Pendias A., Piotrowska M., Lekan Sz., Gałczyńska B., Dudka S., Kacperek K.; 1987. Rolnicza przydatnosc odpadów przemysłowych i komunalnych. [Usefulness of industrial and municipal wastes for agriculture]. Wyd. IUNG Puławy; S(58): 5-182; [in Polish].

11. Kononowa M.; 1968. Substancje organiczne gleby. [Soil organic substances]. Wyd. PWR i L; ss: 390; [in Polish].

12. Piotrowska M., Dudka S.; 1987. Frakcje metali sladowych w osadach sciekowych jako kryterium ich przydatnosci w rolnictwie. [Fractions of trace metals in sewage sludges as critetion of their suitability for agriculture]. Archiwum Ochrony Srodowiska; 1-2: 65-72; [in Polish].

13. Rozporzadzenie Ministra Srodowiska z dn. 1.08.2002 r. w sprawie komunalnych osadów sciekowych. (Dz. U. Nr. 134, poz. 1140). [Regulation of the Minister of Environment on Municipal Sewage Sludge dated 1.08.2002 Jornal of Laws No. 134 item 1140].

14. Skrobiłowicz M.; 2002. Ocena osadów sciekowych z niektórych oczyszczalni województwa podlaskiego pod względem zawartosci substancji nawozowych. [Characterization of sludges from selected sewage treatment plant sof Podlaskie province in respekt of heavy metals content]. Acta Agrophysica; 73: 297-305; [in Polish].

15. Tessier A., Campbell P. G. C., Bisson M.; 1979. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chem.; 51; 7: 844-851.

16. Weber J.; 1993. Wpływ zwišzków próchnicznych na kumulowanie i migrację w glebie niektórych metali ciężkich emitowanych przez przemysł. [Humus substances impact on the accumulation and migration of some heavy metals originated from industry emitted dust]. Zesz. Probl. Post. Nauk Roln.; 411: 283-292; [in Polish].

17. Wójcikowska-Kapusta A., Baran S., Krzywy J.; 2002. Wpływ nawożenia gleby lekkiej osadem sciekowym na wysokosc plonów roslin uprawnych i zawartoœć w nich cynku. [The of the fertilisation of light soil with sewage sludge on the crops of cultivated plants and the content of zinc in them]. Acta Agrophysica; 73: 339-347; [in Polish].

Accepted for print: 07.06.2006

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