IMMOBILIZATION OF HEAVY METALS IN SEDIMENT DREDGED FROM A SEAPORT BY IRON BEARING MATERIALS

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Pot and field trials were carried out using sediment dredged from the port of Bremen (Germany) and deposited in a settling basin near Bremen; the sediment is polluted with Cd and Zn. Five iron-bearing materials were added to the soil (1% pure Fe in soil dry matter) to immobilize the heavy metals: ‘Red mud from the aluminium industry, sludge from drinking-water treatment, bog iron ore, unused steel shot and steel shot waste.

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Nội dung Text: IMMOBILIZATION OF HEAVY METALS IN SEDIMENT DREDGED FROM A SEAPORT BY IRON BEARING MATERIALS

Pergamon War. Sci. Tech. Vol. 37. No. 4-7, pp. 379-384. 1998. 0 1998 IAWQ. Published by Ekvier Science Ltd P&ted in Great Britain. 0273-1223/98 $1900 + O+lO PII: SO273-1223(98)00221-2 IMMOBILIZATION OF HEAVY METALS IN SEDIMENT DREDGED FROM A SEAPORT BY IRON BEARING MATERIALS I. Miiller and E. Pluquet Geological Saxony, Institute of Soil Technology, Friedr. Missler Str. Survey of Lower 46/50,28211 Bremen. Germany ABSTRACT Pot and field trials were carried out using sediment dredged from the port of Bremen (Germany) and deposited in a settling basin near Bremen; the sediment is polluted with Cd and Zn. Five iron-bearing materials were added to the soil (1% pure Fe in soil dry matter) to immobilize the heavy metals: ‘Red mud from the aluminium industry, sludge from drinking-water treatment, bog iron ore, unused steel shot and steel shot waste. The pH and CEC were little influenced by any of these treatments, but the NHdN03 and DTPA extractable amounts of Cd and Zn, and particularly the uptake of Cd and Zn by plants, were markedly reduced. It was demonstrated that red mud and precipitated Fe-bearing sludge were the most effective materials. They caused an increase in the adsorption capacity of the dredged sediment with respect to Cd of about 50%. In the pot trials, NH4N03 soluble amounts of Cd and Zn in samples of soil treated with these materials were reduced by 50% (DTPA -ZO%), while the uptake by plants was reduced by 20-501. In the field trial, Cd and Zn were immobilized in the soil to a certain extent, but less effect was observed on the concentrations in plants and soil extracts compared with the pot trials. In practice, red mud is unsuitable as it contains large amounts of Cr and A13+ ions. Therefore, only sludge from drinking-water treatment, as long as the As concentration in it is low, remains as a useful material for immobilizing heavy metals in polluted sediment dredged from a seaport. 0 1998 IAWQ. Published by Elsevier Science Ltd KEYWORDS Dredged sediments; iron oxides; heavy metals; immobilization; iron-bearing residues; pot trials; field trial. INTRODUCTION Altogether, 700 000 m3/a of fresh sediment has to be dredged from the port of Bremen. Some of the sediment in the River Weser is strongly enriched with heavy metals and is disposed of in settling basins. Not only the total content of heavy metals, but more important the mobile fraction should be known for risk assessment, if the sediment in the settling basin is to be reused or recultivated. One possible remedial measure is to immobilize the heavy metals, thus reducing their mobility and uptake by plants. A common method is to apply dolomitic lime to increase the pH to neutral. Depending on the extent of pollution and the chemical properties of the soil, optimization of the pH alone may not be sufficiently effective in reducing the environmental impact of heavy metals (Ku&e et al., 1984). 319
380 I. MiiLLER and E. PLUQUET Furthermore, the mobile portion of the total heavy-metal content can be reduced by strongly adsorptive materials like clay, organic matter and iron oxides in soils (Brtimmer et al., 1986). The immobilizing effects of an application of clay minerals and organic matter to contaminated soils have been described by Vangronsveld er al. (1995) and Stolzer er al. (1994), respectively. Laboratory experiments have shown that iron oxides adsorb heavy metals from solution (Grimme, 1968) and occlude them (Gerth & Brtimmer, 1983). It has also been verified by electron-microprobe studies that heavy metals in contaminated soils accumulate in iron oxides (Hiller & Brtimmer, 1995). Initial tests treating contaminated soils with iron oxides (Forster et al., 1983; Mench et al. 1994, 1995) have shown that this is a promising way to immobilize heavy metals in soils. Investigations on immobilization of heavy metals have been carried out since 1994 on dredged sediment in a settling basin and two other contaminated sites. Laboratory and pot trials were carried out using five iron- bearing materials to study the changes in mobility and particularly the uptake of heavy metals by plants; the results were then tested in field trials. Several iron-bearing residues were also investigated in the tests. The emphasis of these investigations is on the effectiveness of this method for long term immobilization of heavy metals and its practical application. METHODS Test sites and soils The settling basin investigated is located near Bremen (northern Germany) west of the River Weser, where the latter is joined by the River Hunte. Up to 1992, sediment from the port of Bremen was pumped into the settling basin, forming a layer of sediment 1.80 m thick composed of loamy sand alternating with clayey silt and ribbons of locally pure sand. Having been subsequently drained, the sediment, which is contaminated with Cd and Zn, is now ready to be recultivated without any soil cover. The soil used for the pot trials was taken from a depth of O-15 cm near the field trial plots. This soil showed considerable differences in clay content and therefore in heavy metal content and other pedological parameters (Table l), underlining the typical heterogeneity of sediment in settling basins (Herms er al., 1984). Table 1. Characteristic pedological data of the soil used in pot and field trials soil pH CEC,, C, Fe* Mn* Cd’ Pb’ Zn’ ChY mm01 ____ ---------_--------_mg& ____ ___-_____--___ ____ % % % w-32) lAWlO@ 20.1 7.2 26.9 2.4 2.9 996 4.2 85 4.53 pot trial field trial 24.7 7.1 30.0 3.1 3.2 1150 7.1 145 790 # aqua regia digestion. Potand tests field For the pot trials, soil from the settling basin was sieved; the < 2 cm fraction was mixed with the five selected materials (1% pure Fe in soil dry matter) and placed in 7 L plastic pots with four replicates (Table 2). Watering and application of fertilizer, fungicides and insecticides were held constant. The field trials were limited to treatment with sludge from drinking-water treatment (DW) and iron waste from descaling steel plate (SW) as well as lime treatment and a zero-treatment as control. Each treatment
Immobilization of heavy metals in sediment 381 Table 2. Treatment of soil from the settling basin material used treatment pot trial field trial control (no treatment) co X X x x lime (2 gAc+g soi@ field trial au&: keep pH 7 + 0.5 % free carbonate LI red mud (20 % Fe) from aluminium industry FUVI X DW ‘Fe-bearing sludge (37 % Fe) from drinking-water treatment plant X X BO bog iron ore (38 % Fe) as a natural Fe oxide X SN x new stee[ shot ( 98 % Fe) usu.aQ used as abrake in steel industry SW iron waste (78 % Fe) from SN descaling of untreated steel plate X X The bay barves1 an& the p\arrts used in me pot 1ira\s Iwheat, smnach, rye grass} were an&ys& for heavy metal content and yield. The mobility of heavy metals in the soil samples were determined by extraction with NH4N0, and DTPA. As the investigated settling basin is mainly contaminated with Cd and Zn, this study concentrates on these two elements. The following investigations were carried out on air-dried soil samples sieved to c 2 mm mainly according to German standards: aqua regja digestion lAX!XY&V, )992>, DTEA extraction &h3say & )3oNe)), )9X3), 1 M NH4N03 extraction (E DIN 19730, 1995), cation exchangecapacity (CEC,,,, DIN 19684) and Frew&& i&k~ I&sCd WX~~X+& !k+,+i&&g e> ‘m55uencehabe3> PSJ,anhDUS ]“I &iuc&ti~Y+Y$+Q~ s.nii&ati~i ti Ch Xrratib 0-a 5R80 (lITPA: -20%). Theothertreatments showed impacton theextractable Iess amounts, SW caused a small and inccrease exrracXatieZn. 05
382 I. MijLLER and E. PLUQUET Table 3. pH, CEC,,,, total and extractable Cd and Zn in soil at the end of pot trial using wheat treatment pH Cd Cd CEC, Cd Zn Zn Zn tod DTPA N&NO3 totat DTPA NIX,NOs mm01 ---------------------------------- mg/kg __--------___-______----------------- (CaCM IAE4oog 7.2 23.8 4.2 1.65 0.020 453 86.3 0.31 co LI 7.3 22.8 4.2 1.63 0.014 4.55 83.5 0.28 RM 7.5 24.0 4.0 1.33 0.008 433 71.3 0.12 DW 7.3 27.8 4.0 1.28 0.009 435 69.3 0.12 BO 7.2 25.0 4.0 1.50 0.013 435 77.3 0.25 SN 7.2 22.7 4.2 1.55 0.013 443 80.5 0.26 SW 7.2 23.4 4.1 1.58 0.012 471 99.0 0.37 * aqua regia digestion When it had reached full maturity, the wheat was separated into corn and straw. The yield of corn (CO: 69 g/pot) was only influenced by DW (-7%) and SN (+13%), and the straw (CO: 58 g/pot) by SN (+17%). All treatments caused a reduction of the Cd concentration in both corn and straw by over 30% (Fig. 1). The most effective treatments in reducing the Cd level in corn were red mud (45%), shot waste (-44%) and sludge from drinking water treatment (-41%). The Zn content was influenced less by the treatments than Cd: Except SW, all the treatments caused a decrease in Zn concentration in corn of lo-15% and in straw of about 20- 30%. 60 0.6 70 0.7 60 0.6 0.5 0 . 50 s P 40 0.4 = I i 8 30 0.3 P c 20 0.2 10 0.1 0 0 co LI RM DW 80 SN SW 1 i¶Cdcorn q Cd straw n Zn corn Eal l traw 1 Figure 1. Cd and Zn contents of wheat corn and straw from a pot trial using different treatments. Spinach was used as a test plant from 1994 to 1996, and showed significant variations between the years. The Cd and Zn contents of ‘control’ in 1994 was twice the concentrations in 1995 and 1996 (Table 4). Nevertheless, a declining Cd concentration using RM, DW or SW by IO-20% in 1995 and by 30-50% in 1994 and 1996 was observed over the years. The influence of the treatments on the Zn content of spinach was less uniform. In 1994 LI, RM and SN (all -20%) showed the best results, in 1995 only SW (-15%) and
Immobilization of heavy metals in sediment 383 in 1996 only RM (-30%) were particularly effective. The extractable Cd and Zn fractions hardly differed between the years and equalled the figures in Table 3. Table 4. Cd and Zn in spinach grown over three successive years in pot trials on soil from a settling basin, using different treatments 1996 treatment 1994 1995 Cd Zn Cd Zn Cd zn ----_______----___-------------------------- dry matter mglkg ________________ - ___________________________ co 2.11 255 1.21 104 1.10 111 LI 1.47 191 1.28 99.1 0.95 91.0 RM 1.24 192 0.99 109 0.65 77.4 DW 1.30 240 1.08 93.8 0.69 91.0 BO 1.65 229 1.18 92.5 0.89 92.9 0.97 SN 1.65 202 1.28 97.8 96.7 1.03 263 1.05 85.3 SW 0.81 104 The yield of rye grass grown in pots was only influenced by RM (-10%) and SN (+17%). The Zn concentrations in the grass hardly differ from one treatment to another, whereas the Cd content was reduced by RM, DW, SN and SW by about 2530% (Table 5). Table 5. Weighted means of Cd and Zn contents in grass grown on soil from a settling basin using different treatments treatment pot trial field trial zn Cd zn Cd _________-____________________________---------- ---------- -- ------- -- mg/kgdry mat&r------- - co 0.37 60.3 0.26 73.1 LI 0.33 59.2 0.26 76.3 RM 0.27 64.3 0.26 56.9 0.21 66.3 DW BO 0.30 57.3 SN 0.28 57.5 SW 0.27 64.5 0.18 66.3 Due to the larger total contents of Cd and Zn in the soil used for the field trial (Table 1), the NH4N03 and DTPA extractable fractions were twice those obtained in the pot trials and were reduced by DW and SW by about 10%. Although the Cd content (total and soluble) was much higher in the soil used in the field trial, the Cd concentrations in grass were found to be 20% lower compared with the pot trial (Table 5). The Cd concentration in the grass from the field trial was reduced by treatment with DW (-20%) and SW (-30%) and the Zn concentrations also fell in both cases by about 10%. Correlation between soil extracts and concentration in grass was very low in the field trial, except the recognizible relation between soluble Zn and its concentration in grass (rM.48). In the pot trials, the DTPA and NH4N03 extractable fractions of Cd and Zn hardly differed between the years or between the plants tested, and was almost the same as the data given in Table 3. Correlation between soil extracts and plant uptake was generally poor in the pot trials too, showing two exceptions which were NH4N0, soluble Cd and Cd in wheat corn (r2=0.88) and in grass (r2=0.76). Many authors have found a good correlation between Cd extractable by solutions of neutral salts and Cd uptake by plants (Sauerbeck & Styperek, 1985; Sanders et al., 1987; Prtiess, 1992). Our tests involving
384 I. M%LER and E. PLUQUET treatment of soil with different iron-bearing materials produced a different result, i.e. that there is little correlation between the extractable amounts on the one hand and the uptake by plants on the other. The probable reasons for this disagreement are differences in the distribution of the Fe-bearing material in the soil and changes in structure or crystallinity of the iron-bearing materials when mixed with soil, particularly the formation of iron oxides. These newly formed oxides are not pure iron oxides and may be differently affected by acids released from the roots of plants (Romheld, 1987). Cd adsorotion In laboratory experiments on all the materials, RM and DW demonstrated a large adsorption capacity for Cd. Evaluation of the adsorption data showed that adsorption by the iron-bearing materials followed a Langmuir isotherm, while Cd adsorption by treated soil samples fitted a Freundlich equation. After fitting the Langmuir isotherms to the data (r*>0.98), the adsorption maxima for both the RM and DW were determined as 2000 mg/kg, and those of all other iron-bearing materials being ten times lower. Mixed with the soil these materials gave parallel Freundlich isotherms (r2>0.99). The Freundlich constants (the amount adsorbed at a 1 mg/L concentration in solution) ranged from 180 mg/kg (control) up to 280 mg/kg and 290 mg/kg for RM and DW, respectively (Fig. 2). 3 1 . co . LI . RM . DW 0 so q SW A SN 1 -4 -3,5 -3 .2,5 -2 -1.5 -1 -0.5 0 0,5 Log Cd dhsolved [mgll] Figure 2. Adsorption equilibria of Cd between solid soil and solution phase with fitted Freundlich isotherms using a soil from a settling basin and different treatments. CONCLUSIONS All treatments caused a recognizable reduction of heavy-metal content in plants and soil extracts. But the reduction in Cd concentration in plants was not always enough to achieve a value below the statutory limit. In the pot trials using wheat, the German statutory limit for foodstuffs (BgVV, 1995) of 0.2 mg/kg for Cd was exceeded with all treatments, whereas the Cd concentration in spinach was below half the statutory limit (4 mg/kg dry matter) in all cases. The Cd concentration in grass in both pot trials and field trial was below the statutory limit of 1.14 mg/kg dry matter for animal food (Futtermittel-VO, 1992). Higher Zn concentrations were mainly of phytotoxic interest and only the Zn content in spinach exceeded the phytotoxic value of 150-200 mg/kg suggested by Sanerbeck (1982), but no visible defects or reduced yield were observed.
Immobilization of heavy metals in sediment 385 Special statutory limits for the soluble fraction in Germany are only laid down in Baden-Wtirttemberg (BodSchG BW, 3. VWV, 1993). In the field trial, no treatment was below the limit for NH,NG, soluble Cd of 25 pg/kg, but all treatments in the pot trials were, due to their lower contents of Cd. The statutory limit for NH,NO, extractable Zn, which represents the Zn free to migrate from the soil into the groundwater, (1 mg/kg) was not attained by any treatment in the pot or field test. In the pot trials, all treatments produced results that were below the limit for soluble Zn relevant to animal food (0.5 mg/kg), but in the field trial only DW produced Zn concentrations below this limit. The reasons for the smaller meliorative effects in the field trial can be related back to the fact that the amounts of iron-bearing materials used (1% pure Fe) were too little or that the depth affected (10 cm) was too shallow. This problem is therefore currently being investigated further. The most effective treatments for immobilizing heavy metals in soil from a settling basin of sediments dredged from a seaport were with red mud from the Al industry and sludge from a drinking-water treatment plant. This conclusion was confirmed by our investigations on two other contaminated sites (unpublished). It is questionable whether red mud can be applied in practice, because of its high content of Cr, A13+ and sodium, especially in the soluble form of NaOH (Lotze & Wargalla, 1986). An iron-bearing sludge from drinking-water treatment with a low As content has been demonstrated to be very effective at reducing the heavy metal fraction and uptake of heavy metals by plants. In practice, the treatment should extractable exceed the 1% pure Fe tested and should affect a depth of more than 10 cm. ACKNOWLEDGMENT This work is part of research project No.: 0339601 ‘Reducing mobile heavy metals in contaminated soils by application of iron oxides’, which was conducted with financial support from the Federal Ministry for Education, Sciences, Research and Technology, Germany. REFERENCES AbfKl8rV (1992). Kl~schlammverordnung vom 15.4.1992. Bundesgesetabl. Teil I, 912-934. BgW (1995). Richtwerte filr Schadstoffe in Lebensmitteln; Bundesgesundhbl. 5/9.5,204-206. BodSchG BW, 3. VWV (1993). Dritte Verwaltungsvorschrift des Umweltministeriums Baden-Wthttemberg zum Bodenschutz ilber die Ermittlung und Einstufung anorganischer Schadstoffe im Boden, VWV Anorganische Schadstoffe. Gemeins. Amfsbl. Bad.. Wiirtt., 30, 1029- 1036. Brtlmmer, G., Gerth, J. and Herms, U. (1986). Heavy metal species, mobility and availability in soils. Z. Pjlanzenetihr. Bodenk., 149.382-398. DIN 19684 (1977). FachnormenausschuE Wasserwesen (FNW) im DIN Deutsches Institut for Normung e.V. Teil 8 -Ermittlung der Austauschkapasitiit-, Beuth-Verlag GmbH. E DIN 19730 (1995). Extraktion van Spurenelementen mit Ammoniumnitratlosung. Entwurf, Normenausschutl Wasserwesen (NAW) im DIN Deutsches Institut fdr Normung e.V.. Beuth Verlag. Filius, A., Streck, T. and Richter, J. (1991). Freundlich-Isothermen fur Schwermetalle bei landwirtschaftlich genutzten Be&n. Z angew. Geowiss., H.10, 5-14. Ferster, C., Kuntze, H. and Pluquet, E. (1983). Influence of iron in soils on the cadmium uptake of plants. Proc. 3rd. Inr. Sympos. on sewage sludge, Brighton, Reidel Publ. Comp.(Hrsg.), 426-430. Futtermittel-VO (1992). Futtermittel-Verordnung, Bundesgesetzbl. Teil 1, 1898-1919. Gerth, J. and Brtimmer, G. (1983). Adsorption und Festlegung van Nickel, Zink und Cadmium durch Goethit (-FeOOH). Z. Anal. Chem., 316,616-620. Fresenius Grimme. H. (1968). Die Adsorption van Mn, Co, Cu, und Zn durch Goethit aus verdihtnten LBsungen. Z. PfanzenerMhr. i21,58-65. - Bodeik., Herms, U., Scheffer, B. and Bartels, R. (1984). Schwermetallgehalte in Baden und Pflanzen van Hafenschlick-Spiilfeldem; Fachseminar Baggergut. Veroff. Vortrag v. 27.2.-1.3.1984, Freie und Hansestadt Hamburg (Hrsg.), 143-166. Hiller, D. A. and Brtimmer, G. W. (1995). Mikrosondenuntersuchungen an unterschiedlich stark mit Schwermetallen belasteten BMen. I. Methodische Grundlagen und Elementanalysen an pedogenen Oxiden. Z. Pj7anzenemZihr. 158, 147- Bodenk., 156. Kullmann, A., Macheleit, B., Lehfeldt, J., Starosta, K. H. and Felgetreu, 1. (1982). Zur Wirkung chemisch moditizierten Rotschlammes auf den Schwermetallgehalt einiger Pflanzen und auf die Wasserretention van Sandb6den. Arch. Acker- II. Pflanzenbau u. Bodenkunde, X65-70. Kuntze, H., Henns, U. and Pluquet, E. (1984). Bodentechnologische MaBnahmen zur Sanierung schwermetallbelasteter Spillfelder; Fachseminar Baggergut, Veroff. Vortrag v. 27.2.-1.3.1984, Freie und Hansestadt Hamburg (Hrsg.), 287-307.
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