Evgenios Kokkinos

Optimization of tetravalent manganese feroxyhyte's negative charge density: A high-performing mercury adsorbent from drinking water

This study demonstrates an optimization procedure for the development of an Hg-specified adsorbent able to comply with the regulation limit for drinking water of 1μg/L. On this purpose, the synthesis of Mn(IV)-feroxyhyte was modified to achieve high negative charge density by combining alkaline and extreme oxidizing conditions. In particular, precipitation of FeSO4 at pH9 and excess of KMnO4 follows a very fast nucleation step providing a product with very small nanocrystal size (1-2nm), high specific surface area (300m2/g) and maximum negative charge density (1.8mmol H+/g). The adsorbent was validated for Hg removal in batch experiments and column tests using natural-like water indicating an adsorption capacity as high as 2.5μg/mg at equilibrium concentration 1μg/L under reliable conditions of application. Importantly, the adsorption is an exothermic spontaneous process, resulting in the formation of inner sphere complexes by sharing both A-type and B-type oxygen atoms with the metal surface octahedral as revealed by the X-ray absorption fine structure results.

Mercury removal from drinking water by single iron and binary iron-manganese oxyhydroxides

In this study, single iron oxyhydroxides (FeOOH) and binary iron/manganese (FeMnOOH) oxyhydroxides were used to serve as potential mercury adsorbents. The selection of the optimum adsorbent and the corresponding conditions of the synthesis was based not only on its maximum Hg(II) adsorption capacity but also on its ability to achieve the mercury health regulation limit for drinking water in National Sanitation Foundation challenge water matrix. The experimental results revealed improved adsorption capacity for Hg by the FeMnOOH compared to FeOOH. In addition, the synthesis parameters of FeMnOOH, pH, and redox showed a significant influence on Hg removal efficiency. High redox values and mild alkaline pH improve mercury removal capacity of binary ferric manganese oxyhydroxides.

Nanoparticles for Heavy Metal Removal from Drinking Water

The implementation of nanotechnology in drinking water treatment is a very promising field for applied research. A major part of this effort focuses on reducing the building units dimensions in the existing inorganic adsorbents used for the purification of water versus heavy metal species. The development of engineered nanoparticles has the potential to provide improved uptake efficiencies and sustainability if issues related to cost, technical incorporation and environmental safety will be overcome. We reviewed (1) the technical and economic conditions for potential implementation of inorganic nanoparticles as alternative adsorbents of heavy metals from drinking water, (2) the reported studies referring to the capture of heavy metals ionic forms by inorganic nanoparticles giving emphasis to those succeeding residual concentrations below the maximum contaminant level and (3) the indirect health and environmental risk related to the application of nanosized materials in a water treatment line. In particular, a separate section is devoted to the identification of an optimum nanoparticle profile that fits the unique characteristics of each class of emerging heavy metals with respect to the chemical affinity, charge interactions, aqueous speciation, redox reactions and ion-exchange processes. Importantly, in order to bridge fundamental research with the requirements of the technical and commercial sector dealing with water treatment plants, we introduce an evaluation path for the preliminary qualification of candidate nanoparticulate materials, based on a universal index which is derived by adsorption isotherms recorded under realistic conditions of application.

One step preparation of ZnFe2O4/Zn5(OH)6(CO3)2 nanocomposite with improved As(V) removal capacity

ABSTRACT Novel adsorbents consisting of ZnFe2O4/Zn5(OH)6(CO3)2 (hydrozincite) nanocomposite materials were studied for efficient As(V) removal from water. Nanocomposites were synthesized by the co-precipitation of Zn and Fe salts in alkaline conditions. Depending on the Zn/Fe molar ratio, a variety of materials was produced with different ZnFe2O4/Zn5(OH)6(CO3)2 contents. The adsorbent’s efficiency for As(V) removal was enhanced proportionally to the percentage of Zn5(OH)6(CO3)2 content. The nanocomposite with 74 ± 7 wt% of Zn5(OH)6(CO3)2 provided a capacity of 18.4 μg As(V)/mg for residual concentration of 10 μg/L (pH 7) which is over twice that of an iron oxy-hydroxide prepared under similar conditions.