Remediation of sandy loam soil contaminated with copper by washing assisted with ultrasound

Amongst heavy metal pollutants bound to soil constituents, copper is one of the most widely used, due to industrial and agricultural applications. This study investigated the combination of leaching with biosourced reagents with 45 or 358 kHz ultrasonic (US) irradiation. A soil was artificially contaminated with Cu and decontamination was performed on this unseparated soil and on two of its granulometric fractions: fine loam and fine sand. Mechanisms of dissolution/diffusion were modelled using parabolic equation. Silent washing of the unseparated soil with 0.2 M citric acid led to desorption of 61.9 % of the Cu after 1 h, similarly to 0.1 M HCl leaching. Under 358 kHz or 45 kHz US, faster calcite dissolution and diffusion occurred, leading to higher extraction yield (65.5 %). Citric acid enhanced dissolution by complexing Cu²⁺, but also Ca²⁺ and Mg²⁺ in fractions rich in clay minerals such as loam. Humic acid sodium salt at optimized alkali conditions may also mobilize Cu in alkali conditions. Sonication improved the extraction yield from the unseparated soil from 23.6 % in silent conditions to 30.5 % at 45 kHz and to 37.6 % at 358 kHz. Its effect was particularly pronounced on fine sand, with a yield increase from 18 % to 43 %. Sonication would trigger deagglomeration of bigger particles, and promote the transport of Cu-containing particles by micelles formed by humic species. This study highlighted the potential of high-frequency US in enhancing Cu extraction from a real soil, to a higher extent compared to lower frequency 45 kHz US.