Jong-Chan Yoo

Arsenic speciation and bioaccessibility in arsenic-contaminated soils: sequential extraction and mineralogical investigation.

In this study, a combination of sequential extraction and mineralogical investigation by X-ray diffraction and X-ray photoelectron spectroscopy was employed in order to evaluate arsenic solid-state speciation and bioaccessibility in soils highly contaminated with arsenic from mining and smelting. Combination of these techniques indicated that iron oxides and the weathering products of sulfide minerals played an important role in regulating the arsenic retention in the soils. Higher bioaccessibility of arsenic was observed in the following order; i) arsenic bound to amorphous iron oxides (smelter-2), ii) arsenic associated with crystalline iron oxides and arsenic sulfide phase (smelter-1), and iii) arsenic associated with the weathering products of arsenic sulfide minerals, such as scorodite, orpiment, jarosite, and pyrite (mine). Even though the bioaccessibility of arsenic was very low in the mine soil, its environmental impact could be significant due to its high arsenic concentration and mobility.

Step-Wise Extraction of Metals from Dredged Marine Sediments

In this study, step-wise extraction using various extracting agents was carried out in order to enhance the removal of metals from contaminated dredged marine sediment. Five-times repeated extraction using HCl could extract approximately 23% of the As from the sediment, while NaOH extracted 14% of the As with a single extraction. An alternate application of EDTA and HCl extracted 17%, 34%, 9%, 87%, and 41% of the Cd, Cu, Ni, Pb, and Zn, respectively. Step-wise extraction reduced the risk index (RI) of heavy metals to 49% compared with untreated sediments, which were caused mainly from the decrease in the Cd concentration after chemical extraction in sediment.

Ferric-enhanced chemical remediation of dredged marine sediment contaminated by metals and petroleum hydrocarbons

Sediments nearby harbors are dredged regularly, and the sediments require the stringent treatment to meet the regulations on reuse and mitigate the environmental burdens from toxic pollutants. In this study, FeCl3 was chosen as an extraction agent to treat marine sediment co-contaminated with Cu, Zn, and total petroleum hydrocarbons (TPH). In chemical extraction process, the extraction efficiency of Cu and Zn by FeCl3 was compared with the conventional one using inorganic acids (H2SO4 and HCl). Despite the satisfactory level for extraction of Cu (78.8%) and Zn (73.3%) by HCl (0.5 M) through proton-enhanced dissolution, one critical demerit, particularly acidified sediment, led to the unwanted loss of Al, Fe, and Mg by dissolution. Moreover, the vast amount of HCl required the huge amounts of neutralizing agents for the post-treatment of the sediment sample via the washing process. Despite a low concentration, extraction of Cu (70.1%) and Zn (69.4%) was done by using FeCl3 (0.05 M) through proton-enhanced dissolution, ferric-organic matter complexation, and oxidative dissolution of sulfide minerals. Ferric iron (Fe3+) was reduced to ferrous iron (Fe2+) with sulfide (S2-) oxidation during FeCl3 extraction. In consecutive chemical oxidations using hydrogen peroxide (H2O2) and persulfate (S2O82-), the resultant ferrous iron was used to activate the oxidants to effectively degrade TPH. S2O82- using FeCl3 solution (molar ratio of ferrous to S2O82- is 19.8-198.3) removed 42.6% of TPH, which was higher than that by H2O2 (molar ratio of ferrous to H2O2 is 1.2-6.1). All experimental findings suggest that ferric is effectively accommodated to an acid washing step for co-contaminated marine sediments, which leads to enhanced extraction, cost-effectiveness, and less environmental burden.

Mechanism on Extraction of Heavy Metals from Soil by Ultrasonication

In this study, the mechanisms on ultrasonication enhanced metals extraction were investigated compared with the conventional washing technique. We hypothesized the mechanisms on enhanced extraction of ultrasonication: ultrasonication increased the temperature of soil slurry and decreased average particle size of soil due to breakdown of soil aggregate. Actually, the ultrasonication increased the temperature of soil slurry to 60 o C in this study, and the increase in the temperature enhanced the metal extraction to 15-20% even in the conventional simple mixing. The conventional washing technique decreased average size of soil particles because of breakdown of soil aggregate, and the ultrasonication decreased the size more than that of washing. The breakdown of soil aggregate improved the contact between metals and washing agent, which enhanced the extraction of metals in the ultrasonication. Therefore, we concluded that the main mechanisms of ultrasonication are increase in the temperature and breakdown of the soil aggregate. Finally, the ultrasonicaiton increased the extractability of metals upto 40% compared to conventional washing technique.