Lead Removal from Contaminated Shooting Range Soil using Acetic Acid Potassium Chloride Washing Solutions and Electrochemical Reduction

Background. Cleanup of soils contaminated with toxic metals is a difficult task due to the method inefficiency and the destructive nature of clean-up techniques on soil ecosystems. Objectives. This study was performed to improve the removal efficiency of an acetic acid washing solution for the removal of lead (Pb) from soil. Acetic acid was used in combination with different concentrations of potassium chloride. In order to maximize the removal of Pb from the leachate, different electrode combinations were applied to the washing solutions. Methods. Acetic acid/potassium chloride washing solutions and electrochemical reduction were applied to lead-contaminated soil obtained from an impact berm of a major military shooting range in Ibadan, southwestern Nigeria. The soil was subjected to 5% acetic acid/5% potassium chloride (KCL) and 5% acetic acid/10% KCL solutions in an ex-situ batch experiment. The leachate was electrochemically reduced using 12 volt direct current with a current of 7 amps and 2.5 amps, with aluminum (Al)-Al, iron (Fe)-Fe, Al-Fe and Fe-Al electrodes. Results. The 5% acetic acid/5% KCL proved more efficient for Pb removal in soil with values ranging from 74.9% to 86.9% for 3% soil pulp densities with one washing time of 6 hours. Removal efficiency of Pb from the contaminated soil significantly decreased as the soil pulp density increased. The Al-Al and Al-Fe bipolar electrode combinations showed better removal efficiency of Pb from the leachates with values of 93.7% and 95.6% for 7 amps and 94.5% and 97.3% for 2.5 amps, respectively. Conclusions. The combined 5% acetic acid and 5% potassium chloride washing solution enhances the removal efficiency of Pb in soil and poses less risk to the soil ecosystem and the environment in general.


Introduction
Soil contaminated with lead is a major concern all over the world. This is due to its persistence and toxicity. Lead is known to cause health problems such as cognitive impairments, behavioral disorders, stroke and even death. 1 Different soil decontamination methods have been tried such as physical, chemical/soil washing, electrochemical, biological and integrated processes that combine different methods. 2 Among these methods, soil washing (ex situ or in situ) has been one of the most useful treatments because of its rapid efficient remediation process and its low cost in comparison to other remediation techniques. 3-6 Several washing solutions for extracting metal ions have been widely used and generally include acids, bases and chelating agents. 7 Strong acids and chelating agents such as hydrochloric acid, nitric acid, sulfuric acid and ethylenediaminetetraacetic acid (EDTA) have been proven effective for metal extraction. However, strong acids at high concentration are lethal to soil micro-flora and destructive to the physical and chemical properties of soil due to mineral dissolution. 8 In addition, EDTA is expensive and often results in generation of metal ions in the solution phase which require further treatment to remove. 9,10 A combination of acids at low concentrations with different salts like sodium chloride and calcium chloride have shown promising results in lead removal efficiency. In one instance, lead removal was reported to be 75% from shooting range soil by leaching with 1 M sulfuric acid and 4 M sodium hydroxide, along with removal of copper, antimony and zinc. 11 In another experiment, 0.05 M sodiumethylenediaminetetraacetic acid was used to extract lead from soil at a 1:10 soil to liquid ratio, and this resulted in a removal efficiency of 50-70%. 12 Another investigation utilized a 0.01 M EDTA leaching solution to remove lead from soil and achieved a 46-55.7% removal efficiency, whereas a different investigation used a 5.5 mol sodium chloride (NaCl) solution at pH 3.0 to leach 65% of the lead from soil. 9, 13 Acetic acid can form relatively strong complexes with metal ions and is easily biodegradable and environmentally friendly, but it presents a lower This study was performed to improve the removal efficiency of an acetic acid washing solution for the removal of lead from soil. Acetic acid was used in combination with different concentrations of potassium chloride. In order to maximize the removal of lead from the leachate, different electrode combinations were applied on the washing solutions. . Into each of these bottles was added 100 mL 5% KCL and 400 mL 5% acetic acid to give a total volume of 500 mL of washing solution. This weight/ volume represents 3, 6, 9, 12, 15 and 18% soil pulp density (e.g., 90 g/500 mL x (100%) = 18%) and the batch tests were coded S3, S6, S9, S12, S15, and S18 respectively ( Table 2). The bottles were agitated using an endto-end (Edmund Buhler SM 25) mechanical shaker for 2 hours, after which the mixtures were filtered using Whatman (Cat No 1001, 110 mm) filter paper. The soil residue in the filter paper was then washed with deionized water sequentially twice and both supernatants from the initial leaching and washing were combined together and stored for analysis. The agitation process was repeated for 4, 6, 8 and 10 hours intervals on other sample bottles with the same weight/ volume of soil and solution and with the same volume of 100 mL 5% KCL and 400 mL 5% acetic acid washing Table 1

-Heavy Metal Concentrations (mg/kg) in Impact Berm Soils
Etim  Before and after the electrochemical reduction process, 10 ml of the electrolyte was directly analyzed for Pb by AAS.
Removal efficiency of Pb was calculated by the equation:

Equation 2
Removal Efficiency (%) = (C o -C t )/C o X 100 solution. The entire procedure was repeated using a different washing solution of 100 mL 10% KCl and 400 mL 5% acetic acid following the same weight/volume and interval agitating for 2 to 10 hrs. The supernatants were analyzed directly for Pb using AAS (same instrument mentioned above after calibrating using standards) and the percentage removal efficiency of Pb calculated using Equation

Equation 1
Percentage Removal (%) = C 1 V 1 /C s M s X 100 Where, C 1 and C S are the concentrations of Pb in the supernatant (mg/L) and soil sample (mg/kg) determined using AAS, respectively. In addition, V 1 is the volume of supernatant (L) and M S is the weight of the soil (kg) used for the washing experiment.

Research
Where C 0 is the initial Pb concentration (mg/L) before electrochemical reduction and C t is the Pb concentration (mg/L) after the experiment at a given time interval.

Lead Levels in Soil
According to previous studies, the berm soil is reducing in nature (pH 6.78±0.26) and contains high percentage sand (63.3±9.2%) and organic matter (3.03±2.09%) content. 22 Table 1 shows concentrations of Pb, Cu and Zn obtained from the seven soil samples within the impact berm and their distribution in the various particle sizes. It was generally observed that average levels of Pb in each soil sample were much higher than Cu and Zn, a trend consistent with previously reported studies around the impact berm of shooting ranges. 22, 25 This trend was similarly reflected in the composited soil and its fractionated particle sizes.  Figure 1A and B). In the analysis of fractionated particle sizes of the composited soil, it was observed that Pb levels varied significantly, with the composite (24499 mg/kg) and 2.00 mm particle size (23138 mg/kg) having the highest levels, while the other five particle sizes (0.85, 0.425, 0.250, 0.180 and 0.150 mm) also had relatively high, but similar levels. Levels of Cu and Zn were about the same for the various soil particle sizes with ranges of 529-1054 mg/kg for Cu and 96-183 mg/kg for Zn. From the speciation

Figure 2 -Percentage removal efficiency of Pb in fractionated berm soils using 5% acetic acid and 5% KCL washing solutions (A) composite (B) 0.850 mm (C) 0.150 mm.
A B C Research study (Table 3), Pb and Cu levels were more predominant in the carbonate and organic fraction, which is similar to an earlier reported study carried out within the entire range. 22 Zinc was within the exchangeable and organic fractions.  Table 2). It was also observed that increasing soil pulp density, irrespective of washing duration, corresponded to a decrease in the percentage removal efficiency of Pb. Meanwhile, for each soil pulp density, the percentage removal of Pb increased with duration of washing time ( Figure 3).

Lead Removal by Electrochemical Reduction
The results of the electrochemical reduction of Pb from the combined washing solutions using different electrode combinations are illustrated in Figure 4.

Lead Removal Efficiency in Soil
The presence of corroded bullet fragments resulting from abrasion and weathering could account for the high lead concentrations in the soil fractions. Bioavailable fractions of Pb in the soils could also pose a significant environmental threat considering the high concentration of lead. 27 Analysis of variance at an error probability of 0.05 was used to statistically evaluate the reproducibility and efficiency of each washing solution using Microsoft Excel (Redmond, WA, USA). This was done by multiple comparisons of Pb removal efficiency of the composite, 0.85 mm and 0.15 mm soil fractions for 3 to 18% soil pulp densities at each washing time of 2 to 10 hours (

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Pb (i.e., associated with water soluble, carbonates and organic) was assumed to be removed from the soils, since these fractions are most amenable to metal removal by chemical leaching. 29 Acetic acid is a weak organic acid, and therefore, due to dissolution effects, the excess KCL in the second washing solution may partially remove Pb from the crystalline lattice of the soil. 5,15 The percentage removal of Pb in the contaminated soil for both washing solutions was observed to be lower than the 97% reported by Guemiza et al. when using 0.125 M sulfuric acid and 4 M NaCL with a pulp density of 10% at 1 hour washing time, although this value was obtained after three successive acid leachings. 30 However, the result obtained with 5% acetic acid in 5% KCL in this study compares closely with the 75% removal reported by Lafond et al. using 1 M sulfuric acid and 4 M NaCL solution for 10% pulp density, and the 65% removal of Pb using 5.5 mol NaCl/L in 25% (w/w) of soil pulp density maintained at pH 3.0 by Djedidi et al. 11,13 However, in the study by Lafond et al., the results were obtained after just 1 hour of soil washing which may be due to the strong acid used. 11 Using 0.05 M di-sodium ethylenediaminetetraacetic acid, Demir and Koleli obtained a removal efficiency ranging from 50-70% for just 2 hours of washing, which is also comparable with the results of the present study. 12 The results of the 5% acetic acid and 5% KCL were higher than the 47% reported by Oustan

A B
Research reduction process may possibly further increase the removal efficiency of Pb according to the observed trend. Results obtained compared closely with the 94% Pb removal from acid and saline leachates using mild steel electrodes, 57-76% using -2.0 V potential sources, 15 but below 99.9% when using Fe mono-polar electrodes at a current intensity of 3.0 A. 13,33

Conclusions
In conclusion, the lead concentration in soils at the impact berm related positively with the number of corroded bullet fragments in the soil. The concentration of lead was very high in the composited and 2.00 mm soil fraction, which suggests the presence of small weathered fragmented bullet fragments in the soil. The 5% acetic acid and 5% KCL washing solution was the most efficient solution for Pb removal when it was applied with a 3% soil pulp density and 6 hours washing duration. Longer contact times between the soil and washing solution are desirable. In addition, increasing the number of washing steps will further improve removal efficiency. The Al-Al and Al-Fe bipolar electrode combinations showed the best removal efficiency of Pb from the leachates with values well over 90% at current intensities of 7 and 2.5 A. Although acetic acid is known to be less efficient at Pb removal from contaminated soil, the introduction of potassium chloride enhanced the removal efficiency of Pb, and this washing solution may be an alternative to strong acid and chelating agents. The combined 5% acetic acid and 5% potassium chloride washing solution should pose less risk to the soil ecosystem and the environment in general. Further studies are required to determine the effectiveness of much lower strength acetic acid in combination with other salts for lead removal in contaminated soil.