Green immobilization of Cu, Pb, and Cd in contaminated soils using biopolymers: mechanisms and efficacy

The environmentally friendly biopolymers have been widely utilized in geotechnical engineering in recent years to enhance soil structure and mechanical performance. However, their potential in remediating heavy metal-contaminated soils remains insufficiently explored and recognized. This study proposed three biopolymers, xanthan gum (XaG), guar gum (GuG), and gellan gum (GeG), as green soil amendments to evaluate their immobilization efficacy for heavy metal (Cu, Pb, Cd) contaminated soils. To assess the long-term stability and dosage effects, soil samples were treated with biopolymer concentrations of 0.25%, 0.5%, and 1%, and tested after 1, 28, and 90 d of curing. Then, TCLP leaching, DTPA extraction and BCR sequential extraction were conducted to assess the leachability, bioavailability, and speciation of heavy metals in soil. Under optimal conditions, the toxic leaching risk of Cu, Pb, and Cd were reduced by 29.55-32.73%, 14.09-15.26%, and 6.23-11.47%, respectively, while their bioavailability values were decreased by 12.8-31.58%, 5.71-12.01%, and 14.22-16.57%. XaG exhibited continuously enhanced immobilization capacity under high content and long-term curing, whereas the immobilization effects of GuG and GeG were primarily observed at the medium-term curing stage (28d). BCR results further indicated that biopolymers promoted the transformation of heavy metals from acid-soluble fractions to stable fractions such as reducible or residual fractions. The mechanisms underlying heavy metal immobilization by biopolymers mainly involve functional group complexation, electrostatic attraction, and physical entrapment by the gel network. The findings highlight the application potential of biopolymers in the immobilization of heavy metal-contaminated soils.