Leonardite-derived humic substances are great adsorbents for cadmium

Adsorption is an important mechanism to immobilize cadmium (Cd) in soil, for which humic substances have a potential. However, commercial humic substances are either very acidic (pH = 2) or alkaline/Na⁺-enriched, making them less suitable for use in acid and saline soils. Here, we used leonardite to produce humic adsorbents HA (pH = 4.02), Ca-HA (pH = 10.9), and Ca-CPAM-HA (pH = 9.62) by using HCl, CaCl₂, or CaCl₂-polyacrylamide as a flocculant. Their elemental compositions, acidity, and spectroscopic properties were determined, and their Cd adsorption characteristics were assessed by batch kinetic and thermodynamic experiments at environmentally relevant concentrations. Further, HA was mixed with Cd-contaminated soils and incubated for a month to assess its effect on Cd immobilization. Good fitting of kinetic adsorption data into pseudo-second-order model, together with FTIR spectroscopic data, suggested the chemisorption mechanism by forming Cd(II)-carboxyl complexes. The maximum adsorption capacity derived from the Langmuir equation was 129, 114, and 110 mg Cd(II)/g for HA, Ca-HA, and Ca-CPAM-HA, respectively. These values are almost the same on carbon-normalized basis. HA reduced acetic acid extractable Cd by 31% or more. Besides their high propensity for Cd adsorption, humic adsorbents are inexpensive, safe, and beneficial to soil quality.