Effect of electric field frequency on phytoremediation efficiency and properties of Cd-contaminated soil

Electrochemical-phytoextraction is considered as an integrated technology for removal of Cd in contaminated soil. However, there is a lack of systematic studies on the how electric field frequency plant Cd enrichment and soil microbial properties, which has restricted the optimized application of electric field technology. This study investigated the influence of alternating electric current (AC) at frequencies of 50 Hz and 300 Hz on Cd accumulation in willow (Salix spp.), soil Cd availability, and soil microbial properties. Exposure to AC at 50 Hz and 300 Hz resulted in different degrees of increase in the biomass production of leaves, branches, stems, and roots; the concentrations of N, P, and K in willow; increased root volume and root surface area; and promoted photosynthesis in willow. Compared with the control, AC at 50 Hz significantly enhanced Cd accumulation in the above-ground parts and roots of willow by 14.65% and 9.48%, respectively. Moreover, exposure to AC significantly increased soil Cd availability by 8.34% and 19.26% at frequencies of 50 Hz and 300 Hz, respectively. AC at 50 Hz and 300 Hz decreased soil urease activity but increased the activity of peroxidase acid, phosphatase, and microbial activity. This suggests that AC electric field modulate the interactions between Cd, enzymes, and microbes. Although Cd generally inhibit enzyme activity, AC electric field can alter the soil environment, reduce toxic effects, and enhance oxidative stress responses and nutrient cycling, which can mitigate Cd toxicity. Therefore, these results indicate that the AC electric field promoted willow growth, increased soil Cd availability, and altered soil properties, ultimately leading to Cd uptake in willow. In particular, 50 Hz AC exhibited greater impacts on plant Cd enrichment.