COMPARATIVE EXTRACTION OF Mn AND Pb USING EDTA AND CITRATE TO ASSESS THEIR MOBILITY FROM TWO DUMP SITES IN ZARIA METROPOLIS

An estimate of the labile fractions of different trace metals (Mn and Pb) in soil from two dumpsites with complexing agents (EDTA and Citrate) was carried out. The dump sites used for this study receives both commercial and domestic wastes. Physiochemical parameters and the total metal concentrations of the soil was determined, extraction with EDTA and Citrate were used to study the potential metal extraction capacity at different time intervals and the extraction rates of metal released as a function of time (between 0 to 24 hr). The relatively low levels of silt, clay, organic matter and CEC indicates high permeability, hence leachability of heavy metals in the soil and suggest that it might be amenable to remediation by soil washing. The removal efficiency showed that complete solubilization of metals did not occur, as not all the complexing agents added to the soil, was bound to the target metal. This might be due to the presence of other ions such as Ca and Fe which form relatively high stable complexes. EDTA yielded much more than citrate for both metals under consideration. Lead was extracted more than Manganese, which suggest Lead has been more labile in solution than Manganese and EDTA as a stronger complexing agent than citrate. The level of extraction was constant for the period of 60 min to 720 min after which it increased considerably.


INTRODUCTION
Industrial development and urbanization have led to serious increase in the amount of solid wastes frequently discharged into the natural environment. This has increased the amount of many chemical substances in the environment. The chemical substances which are of great environmental interest are the heavy metals. Heavy metals are of special interest because of the major risks they contribute to the environment, they enter human body through the food chain, causing serious hazards to lives (Liu et al., 2020). Heavy metals are toxic to humans as well as other organisms; they are released by metal-bearing soil constituents and migrate through the soil solution downward to the water table (Van Oort et al., 2006;Shina and Alok, 2010). The contamination by these metals is a threat to the quality of groundwater, if these metals are not properly treated. Unlike organic compounds that can be biodegraded with time or can be incinerated, metals are robust and remain a potential threat to the environment and human health for a long time (Hong et al., 2002). The concentrations of these heavy metals have been found to increase along time. Soil and sediments are rich in these heavy metals since they are not subject to degradation phenomena (Di Palma and Mecozzi, 2007).
The environmental impact of soil contamination depends not only on the total amount of metals in the soil but mainly on their mobility and availability. This is influenced by leaching and interactions with other components of the ecosystem such as air and water. Soil washing remediation technology is used to remove undesirable contaminants in soil and sediments by dissolving or suspending them in a washing solution (Freeman and Harris, 1995;Moutsatsou et al., 2006;Weihua et al., 2010), and also by concentrating contaminants in small volume of soil through particle size separation (Detzner et al., 1998;McCready et al., 2003). This is based on findings that contaminants tend to bind either physically or chemically to clay, silt or organic soil particles. Chelating agents are most effective extractants, which can be introduced in the soil washing to enhance heavy metal extraction from contaminated soils. The advantages of chelating agent include high efficiency of metal extraction, high thermodynamic stabilities of the metal complexes, and low absorption of the chelating agents to a catalyst (Jerome et al., 2007). In addition, the chelating agents cause only minor impact on the physical and chemical properties of the solid matrix as compared to acids (Lee and Marshall, 2002 ;Chen et al., 2019).
Several types of extractants can be used to extract heavy metals and metalloids from contaminated soil for soil washing technology. The extractants can be acids, chelating agents, electrolytes, oxidizing agents and surfactants (Reddy and Chinthamreddy, 2000;Schramel et al., 2000 andSun et al., 2001), out of which acids and chelating agents are the most used extractive reagents for heavy metal decontamination. Acids washing leads to decreased soil productivity and adverse changes in the chemical and physical structures of soil due to mineral dissolution (Reed et al., 1996). Chelating agents are regarded as more attractive alternatives to acids because they can form strong metal-ligand complexes and are thus highly effective in remediating heavy metal contaminated soil (Kim and Ong, 1998;Wei et al., 2011 andCheng et al., 2020). Hence this study is aimed at estimating the labile fractions of Mn and Pb from dump sites by comparing EDTA and Citrate extractions.

MATERIALS AND METHODS
Two dump sites were selected for this study. The first is located adjacent the new Jos road in Zaria Local Government Area (11 o 4'31.8''N, 7 o 43'31.08E) and the second is located behind the Total Filling station opposite Union bank headquarters in PZ area (11 o 6'1.44''N, 7 o 43'14.16''E) of Sabon -Gari Local Government Area, both in Zaria, Kaduna State, Nigeria. The sites are dump sites which receive both commercial and domestic wastes which are incinerated and composite from the sites are collected and often used as manure on agricultural farmlands. For understanding of overall soil development, behaviour and metal distribution, the GPS was used to mark different points of about 50 cm intervals where soil was collected at 0 to 10cm deep. 1kg of soil sample was collected at five different points using a stainless steel hoe on each of the sites. The soil samples collected at different points were dried at room temperature and a composite was made by pounding and sieving to < 2 mm to remove large particles. The composites form was then stored at room temperature in polythene bag for further experiments.
Characteristic physico-chemical parameters of the soil were determined using standard methods (ASTM, 1985). The properties determined include; particle size, pH, CaCO3 content, Cation Exchange Capacity (CEC) and organic matter content. Total metal concentrations in the soil sample were determined using the flame atomic absorption spectrophotometer after HF/HClO4 acid digestion. Extraction with EDTA and Citrate was conducted to assess the effects of chelating agent and reaction time. The extractions were conducted in a 60 cm 3 plastic bottle. The bottle containing 1 g of soil sample and 10 cm 3 of complexing agent were agitated using an end-over-end shaker at a speed of 120 rpm at room temperature for 30 min. this was repeated at 1 hr, 3 hr, 12 hr, and 24 hr. The solutions was centrifuged at a rotating speed of 400 rpm for 15 min and then filtered. The extractions were carried out in triplicates. 0.05 mol dm -3 EDTA and 0.1 mol dm -3 Citrate solutions was used for the extractions, as it has been found optimal for assessing the maximum extractability of metals (Jerome et al., 2007 andFangueiro et al., 2002). All extracting solutions had their pH adjusted to 6.5, as a pH of 6 guarantee minimal variations of pH during extraction and hence, prevent competitive extraction by H + .

DISCUSSION
Preliminary visual inspection showed that the soil was dark-grey in colour indicating a low amount of humus. Colour is one of the characteristics of soil which tells much about the origin of the soil and its composition (Wuana, et al., 2010). Textual analysis showed the preponderance of sand fraction, followed by clay while silt has the lowest composition, thus classifying the soils as sandy loam. Sandy soils are known to have a poor retention capacity for both water and metals (Masakazu et al., 2008;Wei et al., 2011). The high level of sand indicates high permeability, therefore increased leaching of heavy metals. The soil pH measured for site A and B is within the range for agricultural soil (Wasay et al., 1998;Ann and Clain, 2005;Weihua et al., 2010). Based on this, it means that the soil from these dump sites will not have adverse effect on pH of agricultural soil if used. Soil pH plays a major function in the absorption of heavy metals as it directly controls the solubility and hydrolysis of metal hydroxides, carbonates and phosphates. It also influences ion-pair formation, solubility of organic matter as well as surface charge of some ions, organic matter and clay edges (Tokalioglu et al., 2006). Cation Exchange Capacity (CEC) measures the ability of soils to allow for easy exchange of cations between its surface and solution. Estimated CEC was found to be low for the studied sites, this is because of the low amount of organic matter on the sites. The relatively low levels of silt, clay, organic matter and CEC indicates high permeability, hence increased leachability of heavy metals in the soil and suggest that it might be amenable to remediation by soil washing as also reported by Ehsan et al. (2006) and Atafar et al. (2010). Total metal concentration was however low and did not point to a marked industrial pollution. Generally site B tend to show higher concentrations for the studied heavy metals than site A. The concentrations of extractable heavy metals and extraction yield varied considerably with the nature of complexing agent (Liu et al. 2020). The removal efficiency was enhanced by the addition of a masking agent to stop the reaction of other ions such as Ca and Fe as their presence in the soil also interact with the complexing agent due to high concentrations and relatively high stability of their complexes. The discrepancies observed in the percentage extractions for the heavy metals of interest and the two complexing agents may be due to the soil properties such as, metal distribution and physicochemical forms in the soil. Kirpichtchikova et al. (2006) gave two reasons for lack of correlation between metal extractability and the affinity of the organic ligand for the metal in pure solutions; the first is the competitive binding of other cations such as Fe and Ca to the chelant and the second reason is that the heavy metals are bound unevenly to several constituents in soil having different solubilities. Extraction yield for the complexing agents used shows EDTA having a higher yield than Citrate for all the metals under study, this is in agreement with other works that reports EDTA as a more stronger complexing agent than Citrate (Jerome etal., 2007;Masakazu et al., 2008;Wuana et al., 2010;Wei et al., 2011;Chen et al., 2019;Cheng et al;2020). This observation is explicable by the fact that chelate effect is more in EDTA; this effect is found to confer extra stability on chelates and largely originates from an increase in entropy resulting from an increase in the number of free molecules liberated as the chelate is formed. The size, number of rings, substituents on the rings, nature of the metal and donor atoms is among factors that can affect the thermodynamic stability of the chelate systems (Chao et al., 1998). EDTA's superlative extraction yields can also be attributed to its ability to; (i) complex any metal in soil solution; (ii) its ability to absorb and complex loosely held metal ions; and (iii) its ability to dissolve some minerals containing trace metals and complex the freed metals (Di palma and Mecozzi, 2007;Wuana et al., 2010;Wei et al., 2011, Cheng et al., 2020. Desorption of metals and metalloid from soil is an equilibrium process. Therefore extraction time plays a very important role in soil washing process and determines the optimum contact time for contaminants removal. For the metals under consideration, the results showed that an equal time was observed between 60 min to 720 min before a rise is observe again which might be due re-dissolution of the metals in the washing solution.

CONCLUSION
Extraction with EDTA yielded greater amount of metals from the soil than citrate implying that EDTA is a stronger complexing agent than Citrate. Mn is more bonded to soil than Pb implying that it is a less labile metal hence difficult to be completely removed from the soil.