Απομάκρυνση αρσενικού από υπόγεια νερά με συνδυασμό διεργασιών

Abstract

The problem of arsenic contamination of groundwaters has been under extensive discussion especially during the recent years, because of its adverse effects on human health. Several studies have reported that arsenic is a carcinogen and its effects are primarily due to consumption of arsenic contaminated drinking water at concentrations around 100 μg/L. As(V) can replace phosphate in several biochemical reactions, whereas As(III) may react with critical thiols in proteins and inhibit their activity. In Europe, several countries have to deal with the problem of arsenic contamination of groundwaters, used for drinking water, such as Greece, Finland, Italy, Hungary, etc., whereas in other countries, such as Bangladesh and India, arsenic has been reported to reach levels up to 1 mg/L. The Maximum Contaminant Level (MCL) of arsenic in drinking water has been recently revised by the European Commission. According to the new directive, by 2003 all drinking water supply systems within the European Union would have to comply with the new concentration limit, which has been reduced from 50 μg/L to 10 μg/L (EC, 1998). Recently, EPA decided to move forward in implementing the same standard for drinking water. This standard was also recommended by WHO. Therefore, the research on improving the established or developing novel treatment technologies for removing arsenic from contaminated groundwaters is an emerging issue. The distribution of arsenic species [As(III), As(V)] in natural waters is mainly dependent on redox potential and pH conditions (Tallman and Shaikh, 1980). Under oxidizing conditions such as those prevailing in surface waters, the predominant specie is pentavalent arsenic, which is mainly present with the oxyanionic forms (H2AsO4-, HAsO42-) with pKa= 2.19, pKb= 6.94 respectively. On the other hand, under mildly reducing conditions such as those prevailing in groundwaters, As(III) is the thermodynamically stable form, which at pH values of most natural waters is present as the non-ionic form of arsenious acid (H3AsO3, pKa=9.22). Thus, As(III) may interact in smaller extent with most solid surfaces, therefore, it is more difficult to be removed by the conventional treatment methods, such as adsorption, precipitation etc. Several treatment technologies have been applied for the removal of arsenic from groundwaters, such as coagulation/filtration, ion exchange, lime softening, adsorption on iron oxides or activated alumina, flotation and reverse osmosis. Most of these technologies are not efficient enough for the removal of As(III). Therefore, a pre-oxidation step is usually required to transform the trivalent form to pentavalent. The oxidation procedure is mainly performed by the addition of chemical reagents, such as potassium permanganate, chlorine, ozone, hydrogen peroxide or manganese oxides. Although these reagents are effective in oxidizing trivalent arsenic, they may cause several secondary problems arisen mainly by the presence of residuals or from by-products formation, inducing also a significant increase to operational costs of the methods. In the present study an alternative technology for the removal of both trivalent and pentavalent arsenic species was examined, based on the already established biological iron and manganese oxidation and removal from groundwaters. Iron oxidation is caused by several microorganisms, which are indigenous in most groundwaters, such as Gallionella ferruginea and Leptothrix ochracea. The main product of biological oxidation of iron is usually a mixture of poorly ordered iron oxides often containing significant amounts of organic matter. The intermixing of iron oxides, organic material and bacterial presence, produces complex multiple sorbing solids, which exhibit unique metal retention properties. Arsenic(V) can be removed by direct adsorption or co-precipitation on the preformed biogenic iron oxides, whereas As(III) oxidation by bacteria can take place, leading to improved overall removal efficiency. The objective of the present research was to study the mechanism of As(III) removal during biological iron and manganese oxidation, as well as to establish the optimum conditions for efficient arsenic (III & V) removal, in order to meet the new standard of 10 μg/L. Initially arsenic removal was examined using several filter media, which were previously modified by coating their surface by amorphous iron hydroxides. The method, termed “adsorptive filtration”, was examined under the variation of the major parameters. The pH value of water was found to be the most significant towards the removal of both inorganic forms of arsenic. The optimum pH value for As(V) removal was 5, whereas As(III) was removed more efficiently at pH values around 7. The removal of pentavalent arsenic was more satisfactory than trivalent arsenic, indicating the need for preoxidation. Other parameters were found to affect the treatment efficiency such as the linear velocity and the presence of competitive anions. Above all the choice of filter media was found to be the most significant parameter towards the overall performance efficiency of adsorptive filtration. The application of porous polymeric materials such as polyHIPE and alginate produced much better results than the commercially available polystyrene beads. The results were used to model the treatment operation using the “Bed Depth Service Time” and “Empty Bed Residence Time” models. The application of these models enabled the calculation of specific parameters of system performance. The maximum sorptive capacity for 20% breakthrough accounted for 7.79 μg As/g wet alginate bead, whereas the minimum residence time required to achieve effluent arsenic concentration below 10 μg/L, was found to be 76 sec. The further aim of this work was to apply a combined biological and physicochemical process for arsenic removal. The use of iron oxidizing bacteria was found to catalyze trivalent arsenic oxidation by dissolved oxygen and the total arsenic content was removed by sorption on biogenic iron and manganese oxides. Arsenic removal was more efficient by sorption on iron oxides than on manganese oxides. The method was examined under long-term operation. Approximately 70,000 bed volumes of ground water containing arsenic (60-80 μg/L) were treated, in an operation, which lasted around 10months. During this period, residual arsenic concentration was always below the maximum contaminant level and no problems were arisen, regarding treatment efficiency and operative conditions. The results were studied towards the kinetic of reactions of oxidation and adsorption. The rates of biological oxidation of iron, manganese and arsenic were faster than those reported for physicochemical oxidation, indicating the catalytic role of bacteria. The application of the specific treatment technology offers several advantages towards conventional physicochemical treatment methods. Efficient removal of inorganic arsenic can be achieved without the use of any chemicals for oxidative or sorptive processes. This renders the technique environmental friendly and more economical. The operation does not require monitoring of a breakthrough point, like in other column adsorption processes, as the sorbents (biogenic iron and manganese oxides) were produced continuously by the biological oxidation. This contributes to the combined removal of three major groundwater contaminants, iron, manganese and arsenic.