Iron oxide adsorbers for arsenic removal: A low cost treatment for rural areas and mobile applications

Nanoscale iron oxide particles were synthesized and deposited on porous alumina tubes to develop tubular ceramic adsorbers for the removal of arsenic, which is an extremely toxic contaminant even in very low concentrations. In addition, its natural presence affects rural and low-income populations in developing countries in Latin America and around the world which makes it essential to develop an user-friendly, low energy demanding and low cost treatment technology. The system can be operated with minimal trans-membrane pressure difference and does not require pumping. The support tubes and final membrane have been characterized by surface area and porosity measurements, permeability tests and scanning electron microscopy (SEM) imaging. Arsenic concentrations were determined by ICP-OES. This easy to use and low cost process is effective in the removal of arsenic and may provide a valuable solution to the groundwater quality issue in Latin America.

It was found that 350 C is the optimal sintering temperature since it is the lowest temperature at which the iron oxides are fully transformed into hematite, while the particles are successfully bonded to the support. At this temperature, the average pore size and BET specific surface area were measured: 75 nm and 30 m2/g. In addition, the batch experiments have provided an estimate of the material adsorption capacity Fig. 9. Results of the supported membrane setup experiment: (a) arsenic solution filtration; (initial concentration: 86 ppb, pressure: 0.5 bar, flow rate: 1 L/h) and (b) membrane regeneration. Table 2 Permeability of alumina ceramic tubes Sample Untreated tubes Permeability of Al2O3 ceramic tubes Type A tube 916 + 251 Type B tube 8740 + 1842 Sintered with Fe2O3 575 + 309 2935 + 579 of 0.13 mg As/g Fe oxide. On the other hand, the filtration experiments with the alumina supported membrane indicated a higher adsorption capacity value (3.16 mg As/g Fe oxide) for that same level of initial concentration (10 ppm). The apparent higher adsorption capacity of the material as measured in the filtration scheme may be an indication that the batch adsorption of arsenic into unsupported iron ceramics is kinetically limited. In the batch experiments, asprepared iron oxide ceramic (unsupported) was broken into approximately 1 mm particles and stirred in the arsenic solution. While part of the area is readily available for the ions to adsorb, there is an important inner surface area where the arsenate can only access by diffusion, making it the limiting step in the adsorption process. In fact, it was observed that the measured adsorption of the iron oxides vary with initial arsenic concentration, which suggest that the higher driving force given by a higher concentration gradient between the pore volume inside the particles and the bulk solution helped the diffusion step and therefore showed a different pseudo-equilibrium state. This is consistent with other reports that show two different diffusive stages in anion adsorption: a first rapid one in which diffusion occurs mainly in readily accessible sites and a second slower one in which the adsorbing species diffuse into particle aggregates, crystal micropores or are rearranged into surface complexes [3]. In fact, filtration may turn previously inaccessible sites into readily accessible sites, enhancing the rapid initial stage of the adsorption process compared with the batch operational scheme. Mohan and Pittman compared the adsorption capacities of various adsorbents for arsenic removal [6]. Table 2 summarizes the capacity of As(V) removal for iron adsorbents. The adsorption capacity results are comparable to other technologies with similar surface area values. In addition, while other adsorbents present smaller particles with higher adsorption capacities, they are usually related to higher energyassociated manufacturing costs. Regarding the supporting material, its adsorption capacity was measured to be taken into consideration in the different setups. The tubes were coated with an external layer in order to optimize the iron oxides retention and they proved to have an acceptable porosity and permeability. Finally, the recuperation of the membrane’s adsorption capacity was initially studied by performing filtrations with a basic solution that did not cause iron oxide dissolution damaging the membrane. Although the regeneration efficiency obtained seems low, it is probably due to errors of the method and the measurement instruments originated from the proximity to the detection limit. The cost in US dollars of the iron oxide material is 0.03 cents per gram, calculated from the ferrous chloride alone and excluding energy and other reactants. If we consider a daily consumption of 2 L per person, treating a 100 ppb water (which is representative of many rural areas in Argentina) would cost 0.12 cents perday. However, the overall process cost can be significantly diminished given the fact that the iron oxide treating capacity can be regenerated. Moreover, the alumina tubes suffer no deterioration whatsoever and can be used indefinitely unless physical damage is observed.