The use of crassulacean acid metabolism (CAM) plant extracts for the removal of copper from contaminated soils: A sustainable soil washing alternative

Abstract

Extensive research to date has focused on the remediation of heavy metal-contaminated soils by various soil washing techniques. Many of the most effective techniques, however, rely on inorganic acids and/or chelating agents that can result in the deterioration of soil quality and other secondary environmental impacts. In this paper, a new environmentally friendly method for heavy metal-contaminated soil remediation, based on the invasive Opuntia ficus-indica (L.) Mill. is evaluated. The method involves the use of extracts prepared from O. ficus indica cladodes, harvested at phases I (Night) and III (Day) of the plant's photosynthetic cycle, as a soil washing agent. These green extracts were evaluated for the removal of copper (Cu) from three different soils including kaolinite, montmorillonite and a field-natural soil sample. Results indicated that undiluted night phase extracts yielded the greatest Cu removal rates in all three soils with an optimal washing duration of 24 h. Copper removal rates ranged from 56% to 85% and were significantly impacted by the intrinsic properties of the soils, the leachate dosage, the plant harvest phase, the number of washing cycles and the soil washing duration. The chemical characterization of the leachates indicated that the organic acids present in the O. ficus indica extracts as well as other functional groups are key players in the removal process. The obtained results demonstrated the potential of this plant extract for a sustainable abatement of heavy metal-contaminated soil.