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. H2SO4 and H3PO4, 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.