Assessment of pilot-scale acid washing of soil contaminated with As, Zn and Ni using the BCR three-step sequential extraction.

This study performed pilot-scale washing of soil contaminated with both oxyanion and cations as a recalcitrant remediation case due to their different chemical behavior. The soil contaminated with As, Zn and Ni, partially recalcitrant due to their strong binding properties, was obtained near a closed iron/serpentine mining area. This study monitored the variation of chemical speciation of As, Zn and Ni for acid solutions and particle size fraction using the BCR sequential extraction and evaluated the optimal condition of physical separation of highly contaminated fine particles for enhanced washing. H(2)SO(4) and H(3)PO(4), including competitive oxyanions, enhanced removal of As with the simultaneous extraction of Zn and Ni. Less nickel from the residual fraction in coarse particles was extracted than As and Zn due to the recalcitrant serpentine. Fe/Mn oxide, organic/sulfides and residual fractions in fine particles were enriched with contaminants due to the high surface areas and recalcitrant minerals. The chemical extraction of As was also restricted in the fine particles, whereas the chemical extraction of Zn and Ni was determined by the residual form of various particle size fractions. Further extraction was limited in the exchangeable and residual fractions and retained a gradual extraction from Fe/Mn oxide and organic/sulfides fractions, which indicated an instant detachment from the easily bound fraction. Correspondingly, extraction from the acid-attackable fraction was related to the exchangeable Fe/Mn oxide and organic/sulfides fractions. Due to the limitation of chemical extraction, the physical separation of fine particles could enhance the effectiveness of acid washing. In addition, the chemical properties of the soil were affected by strong acid washing. The treated soil then needed to be regenerated.

Investigation and risk assessment modeling of As and other heavy metals contamination around five abandoned metal mines in Korea.

Tailings, agricultural soils, vegetables and groundwater samples were collected from abandoned metal mines (Duckum, Dongil, Dongjung, Myoungbong and Songchun mines) in Korea. Total concentrations of arsenic (As) and heavy metals (Cd, Cu, Pb and Zn) were analyzed to investigate the contamination level. Several digestion methods (Toxicity characteristics leaching procedure (TCLP), synthetic precipitation leaching procedure (SPLP), 0.1 N/1 N HCl) and sequential extraction analysis for mine tailings were conducted to examine the potential leachability of As and heavy metals from the tailings. The order of urgent remediation for the studied mines based on the risk assessment and remedial goals was suggested. The Songchun mine tailings were most severely contaminated by As and heavy metals. Total concentrations of As and Pb in the tailings were 38,600-58,700 mg/kg (av. 47,400 mg/kg) and 11,800-16,800 mg/kg (av. 14,600 mg/kg), respectively. Agricultural soils having high As concentrations were found at the all mines. Average concentrations of Cd in the vegetables exceeded the normal value at all mines areas, while As only at the Dongjung, Myoungbong, and Songchun mine area. One groundwater sample each from the Dongil and Myoungbong mines, and 4 groundwater samples from the Songchun mine had values above 10 mug/L of As concentration. The TCLP method revealed that only Pb in the Songchun tailings, 6.49 mg/L, exceeded the regulatory level (5 mg/L). Employing the 1-N HCl digestion method, the concentration of As in the Songchun mine tailings, 4,250 mg/kg, was up to 3,000 times higher than its Korean countermeasure standard. Results from the sequential extraction of As in the tailings showed that the easily releasable fraction in the Myoungbong and Songchun mine tailings was more than 30% and the residual fraction was less than 40%. Based on results showing the exposure health risk employing the hazard quotient and cancer risk of As, Cd and Zn, the Dongil mine needs the most urgent remedial action. The concentration reduction factor (CRF) of As in both soil and groundwater follows the order: Songchun>Dongjung>Dongil>Myoungbong>Duckum mine.