Adsorption And Reduction Behavior Of Cr(Ⅵ) In Water By Fe Hydrogen Oxides-Fulvic Acid Complexes

As the pace of industrial development accelerates in China,the repercussions of water contamination from an array of heavy metals are intensifying.Chromium,a heavy metal typical of water pollutants,poses a significant threat to aquatic ecosystems.The migration patterns and transformation behaviors of Cr（Ⅵ）within environmental systems are intricately intertwined with the composition of both soil and water-bearing media,encompassing both organic and inorganic constituents.However,the interfacial behavior of Cr（Ⅵ）on composite organic-inorganic materials remains a subject of limited understanding.In this study,ferrihydrite（Fh）and magnetite（Mt）were selected as representative inorganic iron（hydro）oxides,and fulvic acid（FA）was chosen as a representative of natural organic matter.By controlling the addition of fulvic acid,two types of iron（hydro）oxide-organic matter composites with different C/Fe ratios（Fh-FA and Mt-FA）were prepared in the laboratory.Macroscopic adsorption batch experiments were conducted,combined with several characterization analysis techniques,to explore the adsorption behavior of Cr（Ⅵ）on different composites and elucidate the adsorption and reduction mechanisms of the iron（hydro）oxide-organic matter composites for Cr（Ⅵ）.This work aims to refine the binding mechanism of minerals-organic matter-heavy metals,providing a theoretical basis for the remediation of heavy metal water pollution and risk assessment of water environments.The main conclusions obtained are as follows:（1）The structure and morphology of the Fh-FA and Mt-FA composites with different C/Fe ratios prepared in the laboratory were characterized using BET,SEM,XRD,and FT-IR techniques.The BET results showed that the specific surface areas of both composites were affected by the initial C/Fe ratio,with the surface area decreasing as the initial C/Fe ratio increased.The specific surface area of the Fh-FA composite was larger than that of the Mt-FA composite.SEM results revealed slight differences in morphology between the two composites.The Fh-FA composite exhibited an amorphous state with blocky structures adorned with spherical nanoparticles,while the Mt-FA composite showed a more regular cubic structure with smaller spherical particles attached.The surface morphology of both composites was also affected by the C/Fe ratio,with an increase in the C/Fe ratio resulting in more spherical nanoparticles attached to the surface of both composites.FT-IR results indicated that the surface functional groups of the two composites were distinct,each containing unique functional groups.However,the Fh-FA composite contained a wider variety of functional groups than the Mt-FA composite.Changes in the C/Fe ratio did not affect the functional group variation in either composite.（2）The adsorption-reduction efficiency of Cr（Ⅵ）is substantially influenced by C/Fe ratios and pH values.Optimum adsorption of Cr（Ⅵ）by Fh-FA and Mt-FA complexes,reaching 5.81mg/g and 6.38 mg/g,respectively,is achieved when the C/Fe ratio is 0.5.Conversely,the least adsorption of Cr（Ⅵ）,2.97mg/g and 3.26 mg/g by Fh-FA and Mt-FA complexes respectively,is observed at a pH of 7 and C/Fe ratio of 1.5.A negative correlation is observed between the adsorption of Cr（Ⅵ）by the complexes and pH value.Moreover,the initial C/Fe ratio significantly affects the adsorption of Cr（Ⅵ）by Fh-FA and Mt-FA complexes.As the C/Fe ratio increases,the adsorption of Cr（Ⅵ）by both complexes decreases,demonstrating a negative correlation.This decrease is attributed to the presence of FA on the complex which occupies the adsorption sites on the iron ore surface,thereby inhibiting the adsorption of Cr（Ⅵ）by the complex.Furthermore,the abundant reducing functional groups present in FA considerably enhance the reduction of Cr（Ⅵ）.（3）The kinetic process of Cr（Ⅵ）adsorption by Fh-FA and Mt-FA complexes was evaluated through a kinetic experimental model.The process is divided into three stages:rapid adsorption（0-30min）,slow adsorption（30-120min）,and equilibrium（post-120min）.The fitting data indicate that the correlation coefficient R² of the pseudo-second-order kinetic model is superior to that of the pseudo-first-order kinetic model,suggesting that both chemical and physical adsorption of Cr（Ⅵ）occur on Fh-FA and Mt-FA complexes,with chemical adsorption predominating.The isothermal adsorption behavior of Cr（Ⅵ）on Fh-FA and Mt-FA complexes was analyzed using both the Freundlich and Langmuir models.The results favored the Freundlich model for the isothermal adsorption of Cr（Ⅵ）by Fh-FA and Mt-FA complexes,with R² all exceeding 0.9,suggesting that Cr（Ⅵ）adsorption on the two complexes results from the multi-layer spontaneous molecular effects.（4）SEM-EDS characterization of Fh-FA and Mt-FA complexes post-Cr（Ⅵ）interaction reveals Cr（Ⅵ）adsorption on the solid surface,with a dependence on the initial C/Fe ratio in the complexes.As the C/Fe ratio increases,the amount of Cr（Ⅵ）adsorbed on the complex decreases.XPS analysis of surface elements C,O,Fe,and Cr of the Fh-FA and Mt-FA complexes pre and post-adsorption reaction indicates the emergence of Cr（Ⅲ）signals on the complexes,suggesting that both complexes promote the reduction of Cr（Ⅵ）,with the interaction with Cr（Ⅵ）constituting a simultaneous reduction and adsorption process.Furthermore,the adsorption and reduction of Cr（Ⅵ）on the two complexes are influenced by the C/Fe ratio.At a C/Fe ratio of 0 and in the absence of FA,the Mt-FA complex relies heavily on the redox reaction and adsorption of Fe²⁺and Fe³⁺in Mt for Cr（Ⅵ）adsorption and reduction,while the Fh-FA complex relies primarily on the physical adsorption of Fh.When the C/Fe ratio is 1.0,the physical adsorption of Cr（Ⅵ）on both complexes is inhibited in the presence of FA.Both the iron（hydro）oxides and the FA component in the complex contribute to the adsorption and reduction of Cr（Ⅵ）,with the iron（hydro）oxides serving as the primary adsorption carrier and the FA component playing a crucial role in reduction.