Using Iron-Based Adsorbents for the Removal of Inorganic Ecotoxicants from Aquatic Systems

The study investigates the adsorption of heavy metals (HMs), specifically Cd(II) and Co(II), from aqueous solutions using synthesized iron oxides with layered structures and surface groups of a basic nature: goethite α-FeO(OH), lepidocrocite γ-FeO(OH), and ferrihydrite Fe(OH)₃. The results indicate that these crystalline materials are effective in adsorbing heavy metals within pH ranges typical of natural aquatic environments (7.5). We determined the conditions for extracting heavy metals, examining the effect of pH in aqueous solutions, ionic strength, adsorbent dosage, and major components of natural waters. The results show that the majority of HM ions are extracted within the first 30 min of contact between the aqueous solution and the solid phase of the adsorbents, with adsorption equilibrium being reached for iron-containing adsorbents in approximately 4 h. To account for the relative distribution of HM species in aqueous environments with varying pH values, we calculated the distribution of these species for the concentrations studied in model water solutions, enabling an evaluation of the extraction mechanism. The adsorption of Co(II) and Cd(II) is primarily driven by the interaction of their cationic forms with ferrinol surface groups of the studied iron-containing minerals through chemisorption from aqueous solutions within the investigated pH range. The adsorption limits for goethite and ferrihydrite are nearly identical, while those for lepidocrocite are significantly lower, which can be attributed to their structure and the availability of active adsorption sites. The shape of the adsorption isotherm curves is also quite similar across the entire concentration range. Based on the data obtained regarding the adsorption efficiency of Cd(II) and Co(II), which are among the most challenging cations to remove using adsorption methods, the synthesized goethite and ferrihydrite can be recommended as cost-effective and efficient materials for the purification of natural waters contaminated with heavy metals.