SEQUENTIAL EXTRACTION OF Cu, Cd, Pb AND Zn FROM SOIL AROUND INDUSTRIAL WASTE DUMP SITES IN KADUNA ENVIRON USING SIMPLE AND SEQUENTIAL PROCEDURES. H.A Zakari, D.D. Adams, M Shimbayev, P. Nyam Department of Agric Engineering, Kaduna Polytechnic Abstract The presence of heavy metals in soil around industrial wastes dump sites restricts their use for agricultural purpose. This study attempts to extract Cu, Cd, Pb and Zn from soils around some selected industrial wastes dump sites in Kaduna. Four dump sites were identified namely Kudenda, Kakuri, Gonigora and Dirkaniya. Some agronomic parameters necessary to characterize a soil were determined. The simple sequential procedure was use for the extraction. It was confirmed from the study that the metal content of these metals did not exceed the limit by European standard. Keywords: Cu, Cd, Pb, and Zn, extraction, dump sites. Introduction The accumulation of heavy metals is a growing environmental problem. The mobility of trace metals, their bioavailability and related eco-toxicity to plants, depend strongly on their specific chemical forms or way of binding. Consequently, these are the parameters that have to be determined, rather than the total element contents, in order to asses toxic effects and to study geochemical pathways (Feranandez et al., 2000); chemical specification can be defined as the process of identifying and quantifying different species, forms or phases present in a materials. These species can be defined: (1) functionally, for example those species that can be assimilated by plants, (2) operationally, according to the procedures or reagents used in their extraction and (3) specifically, as particular components or oxidation state of an element. Operationally, the definition of speciation including simple and sequential extraction to relate the species associated with particular phases of the sludges (Davidson et al., 1994). In the study described in this paper, a sequential extraction method was applied for Cd, Cu, Pb and Zn in accordance with the scheme described by Agbenin (2002). Step1: Exchangeable fraction Metals adsorption is related with changes in the ionic composition of the water, which may affect the processes of adsorption-desorption. Step2: Reducible fraction or fraction associated with Fe and Mn oxides. The Fe and Mn oxides acts as cement or are present as nodules between particles or cover the same. The heavy metals are strongly bound to these oxides but are thermodynamically unstable in conditions. Step3: Oxidisable fraction or bound to organic matter. That the metals may be complexed or peptized by the natural organic substances is well known. Soluble metallic forms are liberated when organic mater is attacked in oxidant conditions. Step4: Residual fraction. The residual solids mainly contain primary and secondary solids that occlude the metals in their crystalline structures. In addition, concentration of heavy metals were determined as pseudo-total, DPTA-extractable, and water soluble fraction using mixed acid digestion, DPTA (diethylenetriaminepentaacetic acid), and distilled water, respectively. DPTA extraction provided a chemical evaluation of the amount of metals that are available for plant uptake (Hernanadez et al., 1991). This study attempts to compare sequential extraction and DTPA extraction procedure for metal extraction in soils around wastes dump sites. Materials and methods The sequential extraction method described by Sposito et al (1982) with slight modification (Agbenin and Atin, 2003) was used to determine operationallydefined chemical species of the metals. Five operationally-defined fractions of the metals were 114
removed by this sequential extraction illustrated in a flow chart in figure 1. The metal fractions determined quantity of the entrained solution was determined as in the preceding step. were: IV. Occluded in oxides: After noting the quantity of the I. Water soluble: Five grams of air-dried 2mm sieved soil sample was weighed into a pre-weighed entrained solution as in step (iii), to the soil residue from the 0.5 M NaOH extraction was added 25ml of centrifuge tube, to which was added 25ml of 0.05 M Na 2 EDTA solution. The solution was deionized water. The suspension was shaken for 2hr, centrifuged at 3500 rpm for 10 min and filtered through Whatman No. 42 filter paper under vacuum into a 100-ml vial. The centrifuge tube with the soil was re-weighed to determine the quantity of entrained solution. prepared by weighing 1.86 g of Na 2 EDTA into a 1-L capacity beaker to which 200ml of deionized water was added. The beaker was swirled to dissolve the salt and thereafter made to the 1 L mark with deionized water. The suspension was shaken for 6hr centrifuged for 10 min at 3500 rpm and then filtered through II. Exchangeable: The soil residue from water extraction Whatman No. 42 filter paper into a 100ml vial. This was sequentially extracted by adding 25ml of 0.25 M fraction was designated as the occluded or K 2 SO 4 solution. The suspension was shaken for 16hr, precipitated fraction. centrifuged for 10min at 3500 rpm and filtered into V. a Residual: The residual fraction of the metals was 100-ml vial through Whatman No. 42 Filter paper. The quantity of entrained solution was determined by reweighing the centrifuge tube with the soil. computed by subtracting the sum of fractions (i) (iv) from the respective total metal concentration (Bell et al., 1991; Alva et al., 2000) determined by HCIO 4 - III. Organically-bound: 25ml of 0.5M NaOH solution was added to the soil and shaken for 16 hr. the soil suspension was centrifuged at 3500 rpm for 10 min. the supernatant was then decanted and filtered into a 100ml vial through Whatman No. 42 filter paper and was thus designated organically-bound fraction. The HNO 3 -H 2 SO 4 digestion (Agbenin, 1995). At the end of each extraction period, the individual metals in the filtrates were determined by AAS and corrections were made for the concentration of metals in solution taking into consideration the quantity of entrained solutions in the respective steps outlined in figure I. Soil Soluble Figure 1: Extract with deionized water and shake for 2hr Extract with 0.25 M K 2 SO 4 and shake for 16hr Extract with 0.5 M NaOH and shake for 16hr Extract with 0.05 M Na 2 EDTA and shake for 6hr Total metal determined by mixed digestion minus Sum of metal fractions above A schematic diagram of metal fractionation in soils Water Exchangeable Organically Precipitate Residual Characterization of the experimental sites Agronomic parameters: like ph, Electrical conductivity, moisture content, organic matter content, total nitrogen and total phosphorus. These parameters were determined by standard analytical methods, except in the case of nitrogen, which was determined by elemental analysis. Concentration of the metals Concentration of heavy metals were determined by DTPA-Extraction, as follows the conventional diethylenetriaminepenta-acetic acid (DTPA) chelation method of Lindsay and Norvell (1984) was used to determine the amount of extractable or labile Cd, Cu, 115
Pb and Zn in the soil samples as also used for savanna soils by Agbenin (2001). To give a solution of 0.005 M DTPA, 3.94 g DTPA, 2.94g analar grade triethanolamine (TEA) and 2.94 g CaCI 2 were dissolved in 200 ml deionized water in a 2-L capacity beaker. The beaker was swirled to dissolve the reagents and diluted to the 1.5 L mark. Dilute 0.1 N HCI solution was used to adjust the ph to 7.3 + 0.05 before making solution to mark with demonized water. Fifty grams of 2 mm sieved soil samples were weighed into a polystyrene flask to which 100 ml of 0.005 M DTPA solution were added and shaken for 2 hours. The suspensions were filtered through Whatman No. 42 filter paper into 100-ml volumetric flasks. A 20 ml aliquot of the extract was digested with HNO 3 -HCIO 4 to dryness and the residue was dissolved in 10 ml 0.5 M CH 3 COOH solution Extractable Cd, Cu, Pb and Zn in the extract were determined by AAS. Result and Discussion Characterization of the experimental sites Table 1 physical chemical characteristic of the experimental sites Kudenda Kakuri Gonigora Dirkaniya ph 6.6±0.1 7.6±0.3 6.0±0.1 8.2.0.1 Conductivity (ms/cm) 2.27±0.06 2.81±0.04 2.3±0.1 1.25±0.04 Moisture (%) 80.0±0.4 71.2±0.1 82.3±0.1 87±1 Organic Matter (%) 64±1 43.5±0.8 65±1 26.3±0.7 Total N(%) 4.40 3.27 4.56 1.63 Total P(%) 0.77±0.04 1.67±0.04 1.1±0.2 0.42±0.02 Metal Contents Table 2: The metal content of various dump sites are shown in table 2. Kudenda Kakuri Gonigora Dirkaniya TOTAL CONTENT Cd 1.10±0.07 18.3±0.5 1.14±0.06 11.4±0.4 Cu 204±5 337±10 146±7 167±7 Pb 58±1 167±3 87±1 250±10 Zn 487±28 871±37 458±11 697±23 DTPA EXTRACTION Cd 0.37±0.02 3.1±0.03 0.37±0.02 198±0.1 Cu 33.5±0.8 55±2 22.3±0.2 28.5±0.7 Pb 8.1±0.3 18±1 19.3±0.7 35±2 Zn 132±8 120±3 168±2 80±1 WATER SOLUBLE Cd 0.007±0.001 0.026±0.003 0.013±0.001 0.04±0.004 Cu 2±0.3 5.3±0.1 2.19±0.09 1.5±0.4 Pb 0.016±0.0 2.7±0.2 0.13±0.02 0.14±0.03 Zn 1.36±0.04 0.3±0.03 3.5±0.1 0.33±0.02 The results are expressed as mean ± standard deviation in mg/kg of dry Matter. All the heavy metal values recorded for the four dump sites (table 2) were within the maximum permitted levels according European standard mentioned. The total quantity of Cd in all the studied soils (except the Kakuri sample) was low and long below the legally permitted limits. It is necessary to emphasize the high percentage of DTPA Extractable Cd mainly in the kakuri soil which surpasses 28% of the total Cd 116 content. In the case of the kakuri soil, the amount of Cd available for plant was approximately 18% of the total. Bearing in mind the high total concentration of this element in this soil and its known toxicity, the agricultural use of these soils would be inadvisable because of the potential risk of soil contamination.
The Cu in the soil was rather immobile. The DTPA extracted about 15% of total Cu. The concentration of total Pb in the dump sites varied from 251 mg/kg to 58 mg/kg. The concentration of total Zn in dirkaniya and kakuri is about double that of kedenda and gonigora. Phytotoxicity could be caused by this higher Zn bioavailability. Sequential Extraction Scheme Table3 shows the results of sequential extractions Table 3 metal fractions org. bound precipitate exchangeable. Residual Kakuri Cd 0.115±0.004 0.17±0.07 0.43±0.03 0.299±0.008 Cu 2.2±0.2 1.9±0.3 97±8 70±9 Pb 0.94±0.04 0.63±0.03 45±3 15±3 Zn 114±20 84±3 177±20 34±5 Dirkaniya Cd 2.7±0.4 0.769±0.08 13±1 5.3±0.6 Cu 10±1 2.2±0.3 214±12 61±10 Pb 0.225±0.006 0.9±0.1 14±1 125±9 Zn 94±10 77±7 431±38 177±18 Gonigora Cd 0.169±0.009 0127±0.004 0.53±0.05 0.312±0.007 Cu 2.7±0.2 2±0.2 109±5 26±2 Pb 0.08±0.08 18±2 33±5 35±6 Zn 105±6 99±15 174±18 31±7 Kudenda Cd 0.39±0.06 0.39±0.05 8.0±0.8 2.5±0.4 Cu 1.9±0.2 1.1±0.1 120±9 30±2 Pb 0.81±0.07 1.1±0.1 104±18 156±5 Zn 53±5 24.8±0.5 421±25 74±4 The results are expressed as the mean ± standard deviation in mg/kg of dry matter. Cu is associated mainly with the organic matter, the greater extraction percentage was obtained in the organic fraction of the sites which was to be except because of high affinity of Cu to organic matter. This indicates that Cu is associated with strong ligands and probably occluded in minerals like quartz. Zn showed the greatest degree of mobility as seen for the high proportion of metal extracted in the exchangeable and this result reflect the findings of Alvarez (2002). In the easily assimilate fractions (exchangeable relative high percentage of Cd was obtained for Gonigora which proportion of Cd mobilsed reached about 30%. In Dirkaniya and Kakuri the exchangeable will present a potential contamination risk for any soil due to high content of this metal. If the result obtained with sequential extraction are compare with those obtained with DTPA extraction it could be seen that the phyto availability levels of the metal is similar. Conclusion The result obtained after sequential extraction indicate that Cu and Cd were most abundant in the organic and residual phases, while Zn and Pb showed no dominant chemical phase. It ixs clear from story that sequential extraction provides a valuable information on the mobility of metals in industrial wastes dump sites. Reference Alvarez E.A, Mochon M.C, Sanchez J.C. J and Rodrigyez, M.T (2002), heavy metal extractable forms in sludge from waste water treatment plants), Chemosphere 47, PP, 755 775. 117
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