New remediation process zaps pollutants from soil
Marie Donlon | November 02, 2023Rice University scientists alongside researchers from the U.S. Army Engineer Research and Development Center (ERDC) have devised a new approach for improving soil remediation processes.
To remove the most common pollutants from soil — namely, heavy metals like lead, arsenic, zinc, cobalt, copper, mercury and nickel and organic contaminants such as pesticides and microplastics — the team mixed the polluted soil with nontoxic, carbon-heavy compounds that drive electrical current, like biochar, for instance. The researchers then zapped the mixture with short bursts of electricity, thereby flushing out the organic pollutants along with the heavy metals without producing waste or using water to do so. The team has dubbed the process, which is based on Flash Joule heating, the high-temperature electrothermal (HET) process.
Concept of the high-temperature electrothermal process (HET) for soil remediation. a Schematic of the HET process, combined with vacuum extraction well. The vacuum piping and insulation blanket remain standard to known thermal remediation methods, but in the case of HET, the electrodes provide a rapid voltage pulse for electric heating, rather than long-duration heat injection. The soil is premixed in place, with biochar or other conductive carbon to provide sufficient conductivity. b Schematic showing the removal of heavy metals by reduction and vaporization, and the removal of persistent organic pollutant (POP) by graphitization for PAH. c Current curve at an electric input of 100 V for 1 s. d Real-time temperature curve of the soil sample at an electric input of 100 V for 1 s. e Comparison of the HET with other thermal remediation processes, including thermal conduction heating (TCH), electrical resistance heating (ERH), and steam-enhanced extraction (SEE). While operated at lower temperatures, the latter methods require long treatment periods. Source: Nature Communications (2023). DOI: 10.1038/s41467-023-41898-z
The researchers explained that the electrical pulses heat the soil to temperatures up to 1,000° C to 3,000° C in a matter of seconds. This reportedly turns organic contaminants into nontoxic graphite minerals and toxic heavy metals into vapor, which is collected by extraction pipes.
Further, the process is thought to benefit soil fertility, with researchers explaining that germination rates in remediated soil improved by 20% to 30% during trials of the HET process.
The researchers also suggest that the HET process, which can remove multiple pollutants simultaneously, outperforms current methods of removing pollutants from soil, which are time-consuming, expensive and logistically challenging. Further, some current decontamination techniques — for example, surfactant leaching —create secondary waste streams as well as use up significant amounts of water and electricity.
Calling the HET process much faster than other remediation approaches, the researchers also suggest that technique shows promise in the areas of both critical metals recovery from wastes and heavy metals removal for remediation.
"Certain contaminants might be fine — they're not going to move. Other ones might migrate to groundwater and drinking water sources. Some could end up tainting crops, where you could have toxic heavy metals being drawn up through the roots of plants, etc.," the researchers explained. "Being able to regenerate the soil and put it right back where it was, that's a huge advantage over existing technologies that are out there."
A further effect of the process reported by the researchers is that it leaves soil particle size and overall mineral composition relatively unchanged following administration of the technique.
An article detailing the technique, “High-temperature electrothermal remediation of multi-pollutants in soil,” appears in the journal Nature Communications.