Sofia Tresintsi

Kilogram-scale synthesis of iron oxy-hydroxides with improved arsenic removal capacity: Study of Fe(II) oxidation–precipitation parameters

Various iron oxy-hydroxides were synthesized in a continuous flow kilogram-scale production reactor through the precipitation of FeSO(4) and FeCl(2) in the pH range 3-12 under intense oxidative conditions to serve as arsenic adsorbents. The selection of the optimum adsorbent and the corresponding conditions of the synthesis was based not only on its maximum As(III) and As(V) adsorption capacity but also on its potential efficiency to achieve the arsenic health regulation limit in NSF challenge water. As a result, the adsorbent prepared at pH 4, which consists of schwertmannite, was selected because it exhibited the highest adsorption capacity of 13 μg As(V)/mg, while maintaining a residual arsenic concentration of 10 μg/L at an equilibrium pH 7. The high surface charge and the activation of an ion-exchange mechanism between SO(4)(2-) adsorbed in the Stern layer and arsenate ions were found to significantly contribute to the increased adsorption capacity. Adsorption capacity values observed in rapid scale column experiments illustrate the improved efficiency of the qualified adsorbent compared to the common commercial arsenic adsorbents.

A novel approach for arsenic adsorbents regeneration using MgO

An integrated procedure for the regeneration of iron oxy-hydroxide arsenic adsorbents by granulated MgO is proposed in this study. A continuous recirculation configuration, with a NaOH solution flowing sequentially through the saturated adsorbent (leaching step) and the MgO (adsorption step) column beds, was optimized by utilizing the high arsenic adsorption efficiency of MgO at strong alkaline environments. Experimental results indicated that the total amount of leached arsenic was captured by MgO whereas the regenerated iron oxy-hydroxide recovered around 80% of its removal capacity upon reuse. The improved adsorption capacity of MgO for As(V), which is maximized at pH 10, is explained by the intermediate hydration to Mg(OH)2 and the following As(V) oxy-anions adsorption on its surface through the formation of monodentate inner sphere complexes, as it is deduced from the AsK-edge X-ray absorption fine structure (EXAFS) analysis. In addition to the economical-benefits, corresponding tests proved that the solid wastes of this process, namely spent MgO/Mg(OH)2, can be environmentally safely disposed as stable additives in cement products, while the alkaline solution is completely detoxified and can be recycled to the regeneration task.

Tetravalent manganese feroxyhyte: a novel nanoadsorbent equally selective for As(III) and As(V) removal from drinking water.

The development of a single-phase Fe/Mn oxy-hydroxide (δ-Fe0.76Mn0.24OOH), highly efficient at adsorbing both As(III) and As(V), is reported. Its synthesis involves the coprecipitation of FeSO4 and KMnO4 in a kilogram-scale continuous process, in acidic and strongly oxidizing environments. The produced material was identified as a manganese feroxyhyte in which tetravalent manganese is homogeneously distributed into the crystal unit, whereas a second-order hollow spherical morphology is favored. According to this structuration, the oxy-hydroxide maintains the high adsorption capacity for As(V) of a single Fe oxy-hydroxide combined with enhanced As(III) removal based on the oxidizing mediation of Mn(IV). Ion-exchange between arsenic species and sulfates as well as the strongly positive surface charge further facilitate arsenic adsorption. Batch adsorption tests performed in natural-like water indicate that Mn(IV)-feroxyhyte can remove 11.7 μg As(V)/mg and 6.7 μg As(III)/mg at equilibrium pH 7, before residual concentration overcomes the regulation limit of 10 μg As/L for drinking water. The improved efficiency of this material, its low cost, and the possibility for scaling-up its production to industry indicate the high practical impact and environmental importance of this novel adsorbent.

Comparative study of As(V) removal by ferric coagulation and oxy-hydroxides adsorption: laboratory and full-scale case studies

ABSTRACT This research attempts to evaluate the efficiency of the two main arsenic removal processes, i.e. Fe(III) coagulation and iron oxy-hydroxides adsorption, by combining laboratory-scale experiments and results from operating full-scale water treatment units, located in several places of Greece. The proposed experimental procedure showed that the removal ability of Fe(III) coagulation is proportional to the added iron salt dose and appears to be much more efficient than the adsorption process applied in columns filled with iron oxy-hydroxide solid media. Furthermore, the comparison with the results obtained from large-scale water treatment plants indicated the possibility to predict the required quantities of chemicals’ addition, or adsorbents in scaling-up the laboratory or pilot-scale systems, respectively. However, apart from the effectiveness of each method, the variation in water flow rates, the residual arsenic concentrations, and the specific requirements for operation and maintenance should ...

Kinetic modeling of AS(III) and AS(V) adsorption by a novel tetravalent manganese feroxyhyte.

HYPOTHESIS The purpose of the present work is the development of a kinetic model for the adsorption of As(III) and As(V) onto tetravalent manganese feroxyhyte (δ-Fe0.75Mn0.25OOH), which have been recently proved to be very efficient adsorbent for the particular species. EXPERIMENTS In this respect equilibrium and adsorption kinetic experiments onto this type of adsorbent for As(III) and As(V) were performed. Two sizes of adsorbate particles are tested in order to acquire better insight to the adsorption process. RESULTS The adsorption kinetic curves cannot be described by the well-known adsorption kinetic models so a detailed model that takes into account the structure of the adsorbent particle is developed. The model parameters were extracted by the requirement of agreement between model and experimental results. The batch model developed here is necessary for the development of models for fixed bed adsorption devices in order to exploit the commercial prospects of the particular adsorbent. This work constitutes the first attempt of kinetic study and adsorption model development for the specific very promising adsorbent.

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.