(628a) Adsorption of Arsenic, Metallic Ions and Viral Contaminants In Water Using Nanostructured Iron Oxide Membranes

The capacity of iron oxide nanoparticles to adsorb inorganic contaminants such as arsenic, lead and copper has been well documented. One of the foci of our lab is the development of low-cost ceramic membranes for the removal of arsenic and other contaminants from drinking water of underground origin. For example, this is especially relevant for countries such as India, Bangladesh and Argentina, where large rural populations are exposed to natural arsenic contamination of their drinking water.

These membranes are obtained in a three-step process: firstly, iron oxide (lepidocrocite) is synthesized by oxidizing ferrous chloride at mildly acidic pH. Secondly, the obtained lepidocrocite is treated with an organic acid in order to disintegrate iron oxide crystals into nanoparticles, as well as to obtain a functionalized adsorptive surface (ferroxanes). Finally, the obtained ferroxanes can be dried and ground, or suspended in water in order to be infiltrated into a porous alumina support, before sintering at about 450ºC. Therefore, the obtained ceramic can be used in powder form for batch purification processes, or be sintered onto a porous alumina support so as to be used as an on-line filter.

Recently, researchers´ attention has also been drawn towards the possibility of using iron oxides to filter biological contaminants from drinking water and thus render it safe. Bacteria can be relatively easily removed by microfiltration, but smaller viral particles are usually not retained. Thus, the possibility of using iron oxide nanoparticles to remove viral contamination by an adsorption mechanism would represent an interesting and economically sound solution for the potabilization of drinking water.

We have assayed the capacity of the iron oxide membranes synthesized in our lab to withdraw metallic contaminants (copper, lead) as well as arsenic from prepared solutions of these elements. Batch experiments were perfomed in each case by adding different amounts of adsorbent to Erlenmeyer flasks containing a solution of a given concentration. After 48 hs, samples were taken from each flask in order to draw the corresponding points for the adsorption isotherms. For each contaminant, isotherm curves were obtained for different pH levels.

In addition to batch isotherms, continuous filtration experiments were also carried out. In order to do this, iron oxide particles were previously infiltrated into a cylindrical porous alumina support, and then sinterized to form a single porous ceramic unit. Test solutions were then passed through the filter and aliquots withdrawn for analysis using atomic absorption spectroscopy.

As regards biologic contaminants, and given the present trend to avoid the use of chemicals such as chlorine to make water potable, we tested the ability of the said membranes to remone viruses from water. Bacteriophage P22, having an icosaedral capsid approximately 60 nm in diameter, was used as a model for human enteroviruses. Experiments were carried out in an analog fashion as with the inorganic contaminants, performing batch experiments as well as continuous filtration assays. However, due to the slower kinetics of viral adsorption when compared to the inorganic experiments, a recirculacion setup was used for the continuous filtration assay, and aliquots taken at regular intervals. Thus, the original solution, having an initial concentration of approximately 10⁵ pfu/mL was passed several times through the filter (approximately 4 cycles/hour) using a peristaltic pump in a closed circuit. Phage solutions were assayed in all cases applying the plaque forming unit (pfu) methodology as described in Adams (1959).

For inorganic contaminants the results showed that, as expected, acid solutions favored the adsorption of arsenate anions, while cations such as Pb²⁺ and Cu²⁺ were better retained in basic solutions. The maximum adsorptive capacities obtained in the batch experiments were 12.8 mg Pb²⁺/g iron oxide (pH = 6),  33.3 mg Cu²⁺/g iron oxide (pH 7) and 6.0 mg arsenate/g iron oxide (pH 6). The supported filter showed similarly satisfactory results, partly due to the favorable kinetics of the adsorption process.

As regards viral adsorption, batch results showed that the ceramic nanoparticles effectively removed or inactivated most of the initial viral load in the solution. Kinetics experiments, however, evidenced that viral adsorption is a much slower process than in the case of inorganic ions. This was further confirmed by the fact that in the continuous setup, viral removal was achieved gradually along the 48-hour experiment.

These results show that nanostructured iron oxide membranes are effective for the removal of copper, lead and arsenite ions, hinting at the possibility that other inorganic ions can be similarly retained. The ceramic material is also efficient in the reduction of the viral load of a biologically contaminated solution, although the kinetics of the process can be a limiting factor in this case. Therefore, longer residence times as well as larger filtrating surface and/or filtrate recirculation may be warranted in order to achieve an effective purification from the biological standpoint.