Adsorption of dissolved organic matter (DOM) onto the synthetic iron pipe corrosion scales (goethite and magnetite): Effect of pH

• Understanding major factors that control DOM adsorption onto iron corrosion scales.
• Surface properties of goethite and magnetite was changed due to DOM adsorption.
• Higher molecular weight (MW) DOM fraction adsorbed preferentially than lower MW.
• DOM adsorption study was observed to be highly pH dependent.

This study was performed to investigate dissolved organic matter (DOM) adsorption equilibrium using synthetic iron pipe corrosion scales i.e. goethite and magnetite for different pH values ranging from 2.5 to 10.5. The synthesized goethite and magnetite was characterized by XRD, SEM and BET surface area. SEM micrographs for before and after DOM adsorption study revealed a significant change on the morphology of goethite and magnetite surface. Molecular weight distributions of DOM measured by HPSEC revealed that the higher molecular weight fractions adsorbed preferentially onto goethite and magnetite surface, and it was pronouncedly observed at lower pH. The DOM adsorption data illustrated to be fit well by the Langmuir adsorption isotherm indicating monolayer coverage. The standard Gibb’s free energy (ΔGads∘) changes of adsorption process stating the DOM adsorption onto goethite and magnetite surface was spontaneous under the experimental conditions. However, the maximal adsorption capacity for goethite and magnetite were revealed to be 4.75 mg-C/g and 3.79 mg-C/g respectively at pH 2.5. The DOM adsorption study was observed to be highly pH dependent. Consequently, zeta (ζ  ) potential measurements revealed that surface charge of goethite and magnetite was modified due to DOM adsorption onto their surfaces, and their pH for point zero charge (pHPZCpHPZC) was shifted to lower pH compared to that of in absence of DOM. Nevertheless, it has been suggested that ligand exchange between carboxyl/hydroxyl functional groups of DOM and, goethite and magnetite surface was the leading interaction mechanism for DOM adsorption process. FTIR spectroscopy study was in agreement with the conclusion cited above.

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