2018 Volume 59 Issue 10 Pages 1665-1668
Leaching tests were performed in citrate solution with Fe3+ to investigate the leaching behaviors of As, Zn, Pb, and Cu from tailings, which has been known as soil contaminant sources near abandoned mines. The combination of citrate and Fe3+ was selected based on the previous study, which reported that the oxidants such as Fe3+ was required to remove heavy metals from stable forms like sulfides because citrate could leach metals only in carbonate and oxide forms. The leaching efficiencies of heavy metals except As increased with increasing agitation speed, initial Fe3+ concentration, temperature, but decreasing pulp density; e.g. the efficiencies of Zn, Pb, Cu were almost 100% in the 1 M citric solution with 3 M Fe3+ at 400 rpm and 50°C with 0.25% pulp density within 3 hr, but 7.4% for As. The initial oxidation-reduction potential (vs Ag/AgCl) was reduced from 713.5 mV in FeCl3 solution to 656.1 mV in citrate solution with FeCl3, which would inhibit the dissolution of As because As mineral such as arsenopyrite has high open circuit potential. These results indicate that As could be selectively left in the leach residue using citrate solution with Fe3+.
Fig. 4 The effects of addition of citrate on the leaching efficiency of metals at 50°C and 400 rpm with 3 h leaching time, 1 M Fe3+ and 0.25% pulp density.
There are 2089 abandoned metal mines in South Korea, and it has been reported that about 60% (1268) abandoned metal mines have caused mine hazards such as soil contamination.1) Thus, tailings in the abandoned mines have become one representative source of soil metal contamination, and there are increasing concerns about heavy metal soil contamination in the vicinity of the abandoned mines.2) Heavy metals should be removed from soil because they are not decomposed naturally in the soil.3) Although gravity separation processes have been preferred in Korea for the remediation, it is difficult to treat metal contaminants from tailing of abandoned mines with the process due to their fine particle size.
Since the fine particles have a large specific surface area, the leaching process known as soil washing is suitable for the treatment of fine contaminated soil particles (<75 µm) which has been dumped in landfill sites. Strong inorganic acids such as HCl and HNO3 are effective in extracting heavy metals from contaminated soils, but could acidify the treated soil (pH < 0), which requires additional processes for neutralization.3) Citrate, an organic matter, has been recognized as an alternative leaching reagent due to superior complex formation with heavy metal ions.4–7) Since citrate is biodegradable, there is no risk of remaining after treatment.7)
In the previous study,3) it was reported that citrate could dissolve metals in the form of oxides or carbonates, but not in the form of sulfides or metallic forms. The amount of metal leached by citrate from contaminated soils varies with the amount of weathered form such as oxides and carbonates,3) so appropriate oxidants should be added to remove sulfides from contaminated soils. Ferric ion (Fe3+) has been found to be a strong oxidant (Fe3+ + e = Fe2+, 0.77 V), which could oxidize some sulfides.8,9) Furthermore, the use of Fe3+ has the advantage that it can be reused by oxidizing electrically or biologically Fe2+ produced after the oxidation reaction.8,9)
Generally, metal contents in contaminated soil are low, so it is difficult to examine the leaching behaviors of metals. Therefore, in the present study, leaching tests were carried out by collecting soil sample from the tailing causing soil contamination. The effects of agitation speed, Fe3+ and citrate concentration, temperature, pulp density on the leaching behavior of As, Zn, Pb, Cu were discussed here.
The tailing samples used in this study were collected from an abandoned mine site in Korea. The sample was air-dried and sieved with 75 µm sieve to remove debris and pebble. The chemical composition of the sample, obtained from aqua-regia digestion, was given in Table 1, other metals except Fe, As, Mn, Zn, Pb, and Cu were not detected. As shown in Fig. 1, the XRD pattern indicates that SiO2, FeAsS, FeS2, As2S3 are main minerals in the samples. All reagents used were of reagent grade.
XRD pattern of tailing sample used in this study.
Leaching tests were conducted in a 500 ml three-necked Pyrex glass reactor with the heating mantle to maintain the desired temperature. The reactor was equipped with a stirrer and a reflux condenser to avoid solution loss at high temperatures. The leaching solutions were prepared by using ferric chloride hexahydrate and citric acid monohydrate or sodium citrate tribasic dihydrate, and the concentrations of citrate and ferric chloride were 1 M and 0.5–3 M, respectively. By adjusting the ratio of citric acid and sodium citrate, solution pH was adjusted at 1.5 except for tests to examine the effects of citrate addition and Fe3+ concentration. After 200 ml of the solution was placed in the reactor and the solution was heated to desired temperatures (30°C–90°C) while agitating at 200–600 rpm, a designated amount of sample was added to the solution. After the leaching test, leach residue was collected and digested with aqua regia for XRD analyses and for calculation of leaching efficiency.
2.3 Analytical methodsDuring the leaching test, 1.5 ml of solution was withdrawn with syringe, and was filtered with 0.45 µm membrane filter. The filtrate was diluted with 2% HNO3 solution and, Metal concentrations were then measured with OPTIMA 8300 DV inductively coupled plasma-atomic emission spectrometry (ICP-AES, PerkinElmer Inc., USA). The leaching residue was analyzed with SmartLab X-ray diffractometer (XRD) (Rigaku Co., Japan).
As Fe and Mn are not regulated by the Korean Soil Environment Act, leaching behaviors of As, Zn, Pb, and Cu were investigated in this study. Leaching tests were performed at an agitation speed of 200–600 rpm to investigate the effect of liquid film boundary diffusion surrounding solid particles on leaching efficiencies of metals in 1 M citrate at 90°C with 3 h leaching time, 1 M Fe3+ and 0.25% pulp density. As shown in Fig. 2, the leaching efficiencies increased gradually from 200 rpm to 400 rpm and then remained almost constant until 600 rpm. Thus, in all subsequent leaching tests, an operating agitation speed of 400 rpm was selected to ensure effective particle suspension in the solution while minimizing the effect of liquid film boundary diffusion surrounding the particles. The leaching efficiency of Zn reached 100% at over 400 rpm, and those of Cu and Pb were 91.6% and 85.5% at 400 rpm, respectively, whereas that of As was only 11.2% at 400 rpm.
The effects of agitation speed on the leaching efficiency of metals in 1 M citric acid at 90°C with 3 h leaching time, 1 M Fe3+ and 0.25% pulp density.
In this study, Fe3+ ion was added as an oxidant, and the oxidation of sulfides by Fe3+ could be expressed as follows;
| \begin{equation} \textit{MeS} + 2\textit{Fe}^{3+} = \textit{Me}^{2+} + 2\textit{Fe}^{2+} + S^{0} \end{equation} | (1) |
The effects of initial ferric concentration on the leaching efficiency of metals in 1 M citrate at 50°C and 400 rpm with 3 h leaching time, 0–3 M Fe3+ and 0.25% pulp density.
The effects of addition of citrate on the leaching efficiency of metals at 50°C and 400 rpm with 3 h leaching time, 1 M Fe3+ and 0.25% pulp density.
XRD pattern of leach residue obtained from the result with citrate and ferric ion shown in Fig. 4.
The effects of temperature were investigated in 1 M citrate at 400 rpm with 3 h leaching time, 1 M Fe3+ and 0.25% pulp density. Although the results in Fig. 3 shows the higher Fe3+ concentration yields the higher leaching efficiency, because there is concern about precipitation in the solution with higher Fe3+ concentration, the concentration of Fe3+ was set at 1 M. The leaching efficiencies of metal increased with increasing temperature, where the increase of As was not remarkable as shown in Fig. 6.
The effects of temperature on the leaching efficiency of metals in 1 M citrate at 400 rpm with 3 h leaching time, 1 M Fe3+ and 0.25% pulp density.
In this study, although the leaching behaviors of metals in tailing sample were investigated for the remediation of heavy metal soil contamination caused by mine tailing, generally because the metal contents do not exceed 0.1% in the contaminated soils near the abandoned mines in Korea,1,2,13) the leaching tests were performed with low pulp density. The effects of pulp density (0.25%–1%) on the leaching efficiency of metals were investigated in 1 M citrate at 400 rpm and 90°C with 3 h leaching time and 1 M Fe3+. As shown in Fig. 7, all leaching efficiencies decreased with increasing the pulp density from 0.25% to 1%. This result would be due to a lack of oxidants, and these behaviors show a good agreement with the results of ferric ions as shown in Fig. 3.
The effects of pulp density (0.25%–1%) on the leaching efficiency of metals in 1 M citrate at 400 rpm and 90°C with 3 h leaching time and 1 M Fe3+.
These results suggest that As component could be separated successfully using citrate leaching with ferric ion, but there are still As in the leach residue and heavy metals ion in the leachate. As mentioned above, Fe2+ ion generated after leaching reaction (eq. (1)) is oxidized electrically into Fe3+ ion,8,9) which could be reused in the leaching step. During this process, the metal ions could be removed on cathode electrodes by electrical-reduction, or precipitation and extraction method are also promising options.14) For arsenic in the residue, there have been many studies for removing As from solid materials,15–17) and the selective As precipitation suggested in this study would facilitate the As removal processes from the residue. Further study should be required to confirm the removal of the metals in the future.
The leaching behaviors of heavy metals from tailings, which have caused the soil contamination near abandoned mine sites, were investigated using citrate solution with ferric ion. The combination of citrate and ferric ion enhanced the leaching efficiencies of heavy metals from the sulfide form of metals except As, which could be achieved by complexing and oxidizing powers of citrate and ferric ion, respectively. However, the addition of citrate partially reduced the oxidizing power and limited the dissolution of As. These results indicate that the combination of citrate and ferric ion could leave selectively As in the leach residue, which could be confirmed by investigating the tailings with high level of metals. Therefore, this method would be useful for treating As separately from other heavy metals and for understanding the leaching behavior of As in citrate solution.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B1008174).
