Mechanisms and Effectiveness of Biochar, Zeolite and Attapulgite for Heavy Metal Immobilization in Soils: A Comparative Review

Heavy metal contamination of soils represents a persistent environmental challenge, for which in situ immobilization has emerged as a cost-effective and technically viable alternative to conventional invasive remediation technologies. This review comparatively evaluates three distinct categories of soil amendments—biochar, zeolite and attapulgite—within a unified analytical framework integrating extractable fractions (TCLP, DTPA, and CaCl2) and geochemical fractionation approaches (BCR and Tessier). The novelty of this study lies in the systematic assessment of the dominant immobilization mechanisms associated with each amendment in relation to soil properties and the chemical speciation of the target metal, as well as in distinguishing between an apparent reduction in metal extractability and a genuine shift toward more stable geochemical fractions. The findings identify ion exchange as the primary immobilization mechanism in zeolites (NaA zeolite, 1–5% w/w, 96% reduction in TCLP-extractable Pb and 91% reduction in TCLP-extractable Cd), the synergistic action of adsorption, complexation, and precipitation in biochar systems (manure-derived biochar, 0–5% w/w, 97.4% reduction in the exchangeable Pb fraction according to the Tessier scheme), and the critical role of surface modification in attapulgite-based amendments (C-ATP, 4% w/w, 95.1% and 74.3% reductions in TCLP-extractable Pb and Cd, respectively). Because these efficiencies were obtained using different extraction protocols, they are not directly comparable. At the same time, cases of adverse responses were identified, including increased As extractability following the application of phosphate-modified biochar and the redistribution of Pb and Cd after amendment with natural zeolite in industrially contaminated soil. These observations highlight that amendment performance is not an intrinsic property of the material itself, but rather the outcome of specific geochemical interactions occurring within the soil system. Increased soil pH emerged as the principal common factor promoting metal stabilization across all amendment categories, whereas substantial variability in amendment dosage, incubation period, and analytical methodology limited direct quantitative comparisons among studies. Consequently, the selection of an appropriate soil amendment should be based on the integrated evaluation of soil physicochemical properties, contaminant speciation, and the intended scale of application, supported by long-term monitoring under field conditions.