Due to the high surface reactivity and redox chemistry, iron (Fe) minerals have a strong control on contaminant speciation, mobility and degradation. This has been well established for sediment and solution systems, and this review evaluates the role of Fe minerals in contaminant cycling from a sediment pollution perspective. Sediment redox conditions govern the Fe mineralogy, and a detailed description is given for Fe mineral interactions with contaminants in both oxic and sub/anoxic sediment horizons. These interactions include contaminant immobilisation through adsorption and co-precipitation mechanisms and contaminant degradation and speciation changes caused by Fe redox chemistry. Based on these reductive and adsorptive capabilities, recent advances in Fe amendment technologies, particularly in the field of engineered zero-valent Fe nanoparticles, have shown promising results for the treatment of polluted soils and sediments. However, the variable chemical and physical dynamics of sediment systems remains a limitation to the global application of these technologies.

Hundreds of millions of people are at risk from drinking arsenic (As)-contaminated groundwater in the world, making As removal from aquatic systems of utmost importance. However, characteristics of As removal by bacteria-induced ferrihydrite and coupled with redox processes are still not clear. Two-line ferrihydrite was formed in the presence of aerobic Fe(II)-oxidizing bacterium, Pseudomonas sp. strain GE-1. Arsenic co-precipitation with and adsorption onto ferrihydrite induced by Pseudomonas sp. strain GE-1 and redox processes of As were investigated. Results demonstrated that co-precipitation performed better in As(V) removal than As(III) removal, while adsorption showed higher capacity for As(III) removal. X-ray absorption near-edge spectroscopy (XANES) indicated that As(III) oxidation occurred in solid phases during co-precipitation and adsorption. Detection of As species in solution showed that As(V) was reduced to As(III) during co-precipitation, although no As(V) reduction occurred during adsorption. Arsenic immobilization by Pseudomonas sp. strain GE-1-induced ferrihydrite in the presence of the strains may be applied as an alternative remediation strategy.

Magnetic nanoparticles of CoFe₂O₄ were synthesized by co-precipitation method. The magnetic material silicalite-2@CoFe₂O₄ (SC) was prepared by using tetrabutylammonium hydroxide as the template, tetraethoxysilane as the silica source and CoFe₂O₄ as the magnetic core. TiO₂/silicalite-2@CoFe₂O₄ (TSC) magnetic photocatalyst was prepared by sol-gel technique using SC particles as the supporter and tetrabutyltitanate as the titanium source. The samples were characterized by X-ray diffraction, scanning electron microscopy, N₂ adsorption-desorption, Fourier transform infrared spectroscopy, and ultraviolet (UV)–visible diffuse reflectance spectra. The reduction of Cr(VI) in aqueous solution by UV/TSC process was studied under various operating conditions. The results demonstrate that the as-synthesized TSC has high photocatalytic activity due to the high dispensability of TiO₂ provided by silicalite-2@CoFe₂O₄. The removal of Cr(VI) reached 72.9 % by using 0.6 g/L of TSC under the optimum conditions within 180 min. The photocatalytic reduction of Cr(VI) by TSC followed the Langmuir–Hinshelwood kinetic model. At the end of the reaction, TSC could easily be recovered and could be reused without the significant loss of the catalytic activity.

Scallop shell-Fe₃O₄ nanoparticles were synthesized by co-precipitation and hydrothermal methods. The removal efficiency of RB5 was studied as a function of pH, adsorbent dosage, initial RB5 concentration, ionic strength, and temperature. Coating of Fe₃O₄ nanoparticles onto Scallop shell was identified by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDX) analysis. Maximum adsorption was obtained at pH 3. The removal efficiency of RB5 was increased with increasing adsorbent dosage. However, it was decreased with increasing initial RB5 concentration, temperature and in the presence of any anions. Adsorption kinetic study revealed that the pseudo-second order model better described the removal rate than the pseudo-first order model and intra-particle diffusion model. Adsorption isotherm was analyzed by both Langmuir and Freundlich equation. Experimental result was well described by the Langmuir equation. Maximum adsorption capacity was estimated to be 1111.11 mg/g. Thermodynamic studies indicated that the adsorption of RB5 onto Scallop shell-Fe₃O₄ nanoparticles was an endothermic (∆H = 178.14 KJ mol⁻¹) process. The negative values of free energy (∆G) for the adsorption indicated that adsorption of RB5 was spontaneous reaction. Adsorption activity of RB5 by Scallop shell-Fe₃O₄ nanoparticles was maintained even after six successive cycles.

Fe₃O₄and Fe₃O₄/bentonite were prepared by chemical co-precipitation method. They were characterized by X-ray powder diffraction (XRD), Fourier infrared spectroscopy (FTIR), and transmission electron microscope (TEM). Adsorption of cobalt(II) on the bentonite, Fe₃O₄, and Fe₃O₄/bentonite nanocomposite was studied. The results indicated that the metal oxides mainly occurred in the form of spinel structure of Fe₃O₄and the presence of Fe₃O₄significantly affect the surface area and pore structure of the bentonite. The specific surface area (Brunauer–Emmett–Teller (BET) method) of bentonite, Fe₃O₄, and Fe₃O₄/bentonite were determined to be 34.44, 98.44, and 140.5 m² g⁻¹, respectively. TEM image of Fe₃O₄/bentonite shows the particle diameter at 10 nm. The maximum adsorption capacity of cobalt(II) by Fe₃O₄/bentonite nanocomposite was determined to be 18.76 mg g⁻¹. The adsorption strongly depends on pH, where the removal efficiency increases as the pH turns to alkaline range (pH 9). The results suggest that higher adsorption capacity of composite than bentonite is attributed to the presence of Fe₃O₄. The adsorption process follows pseudo-second-order kinetics. The equilibrium data was analyzed by Langmuir model showing high correlation coefficient. The thermodynamic study of adsorption process showed that the adsorption of Co(II) onto Fe₃O₄/bentonite was carried out spontaneously.

Surface and groundwater are often contaminated with toxic anions such as arsenic and selenium. Because of their large surface areas, selenium adsorption on carbon sorbents is considered an attractive water treatment technique. In this present work, selenium sorption on copper-impregnated activated carbon and fly ash-extracted char carbon was evaluated. Unburned carbon was extracted from fly ash using froth floatation techniques, and the carbon sorbents were modified using copper ions. Adsorption experiments confirmed the strong influence of electrostatic forces on equilibrium uptakes of selenite (Se (IV)) and selenate (Se (VI)). Selenium sorption on virgin char carbon was maximum only at acidic pH, i.e., at pH < pHₚzc (pH at point of zero charge). Upon copper modification of the carbon surface, the pHₚzc shifted towards the alkaline region, and as a result, the positive charge density on the carbon surface increased. At pH > pHₚzc, a two- to fourfold increase in sorption coverage and threefold increase in selenium percent removal was observed. Se (IV) sorption was higher compared to Se (VI) sorption. The effect of selenium concentrations and competing anions was studied to evaluate adsorbent performance. The order of maximum surface coverage followed the order: modified char carbon > modified activated carbon > char carbon. The main mechanism of selenium (Se) sorption appeared to be (1) electrostatic attraction of the Se ions to the modified carbon surface at acidic to neutral pH; (2) complexation of Se ions with the copper ions/oxides on the carbon surface; and (3) co-precipitation with copper hydroxides at alkaline pH.

Extractions with mixtures of oxalic acid, ammonium oxalate, and ascorbic acid have been used to analyze trace metals that are occluded in Fe and Mn oxides in soil samples. In our experiments, the amount of Fe extracted from two Andisols decreased with increased extraction time using a mixture of 0.1 mol L⁻¹oxalic acid, 0.175 mol L⁻¹ammonium oxalate, and 0.19 mol L⁻¹ascorbic acid (OxAsc). Differential X-ray diffraction analysis showed the presence of Fe(II)C₂O₄ · 2H₂O in the residue after the extraction, and the amount of this precipitate increased with extraction time. These results indicate that the decrease in Fe extraction with time was caused by precipitation of Fe(II)C₂O₄ · 2H₂O. The amounts of some trace metals (Cd, Co, Mn, Ni, and Zn) extracted with OxAsc showed trends similar to that of Fe, whereas As, Cu, and Pb did not show this trend. A coprecipitation experiment showed that the concentrations of Cd, Co, Mn, Ni, and Zn markedly decreased after the precipitation of Fe(II)C₂O₄ · 2H₂O, whereas those of As, Cu, and Pb did not noticeably change. These results indicate that some trace metals coprecipitated with Fe(II)C₂O₄ · 2H₂O during extraction with OxAsc. This coprecipitation results in underestimation of oxide-occluded trace metals in soils; therefore, using mixtures of oxalic acid, ammonium oxalate, and ascorbic acid to extract some trace metals occluded in Fe and Mn oxides may yield erroneous data.