Ultrasound to decontaminate heavy metals in dredged sediments

Sediments contaminated with heavy metals due to past disposal practices threaten the environment and require remediation. This study was an attempt to develop a technology to decontaminate heavy metals in dredged sediments using ultrasound coupled with vacuum pressure. A set of laboratory scale experiments were conducted using dredged sediments obtained from New York/New Jersey harbor. This sediment sample is considered as category III, a material that failed to meet USEPA requirements for toxicity or bioaccumulation, and required secure disposal. Acoustic cavitation due to ultrasound energy coupled with vacuum pressure was used to facilitate the removal of chromium (the selected metal contaminant) from the sediments. Full factorial experimental designs were performed to evaluate the above treatment technique and to optimize the processes. Two coupled processes were used to separate and to treat both coarse (Process #1) and fine (Process #2) fractions of sediments. Selected variables for evaluation of Process #1 were ultrasound power, soil-to-water ratio, vacuum pressure and dwell time, and for Process #2 were ultrasound power, soil-to-water ratio and dwell time. Laboratory scale experiments were carried out with various combinations of these parameters according to the factorial design. The optimum removal was found to be 92% with the parameter levels at 1200 W power, 1:15 soil-to-water ratio, 15 psi vacuum pressure and 15 min of dwell time. After the application of Process #2 for fine sediments it was found that the fines were separated into silt and clay. Only the silt faction had a considerable metal removal while the clay fraction was insensitive to the treatment. A maximum removal of 83% was obtained for silt fraction when factor levels were at 1200 W power, 1:50 soil-to-water ratio and 90 min of dwell time. Further analysis of clay fraction showed that the chromium in clay is immobile and stable. The toxicity characteristic leaching procedure (TCLP) test on clay fraction confirmed that the treated clay fraction is safe to dispose. The study showed that the proposed treatment technique is effective and economical for sediments with lower clay contents.