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A PILOT STUDY FOR ARSENIC REMOVAL FROM WATER BY ADSORPTION IN NATURAL ZEOLITE ADSORPTION IN PRESENCE OF IRON AND MANGANESE.

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A PILOT STUDY FOR ARSENIC REMOVAL FROM WATER BY ADSORPTION IN NATURAL ZEOLITE ADSORPTION IN PRESENCE OF IRON AND MANGANESE. M. L. RIVERA and M. PIÑA Instituto Mexicano de Tecnología del Agua Paseo Cuauhnáhuac
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A PILOT STUDY FOR ARSENIC REMOVAL FROM WATER BY ADSORPTION IN NATURAL ZEOLITE ADSORPTION IN PRESENCE OF IRON AND MANGANESE. M. L. RIVERA and M. PIÑA Instituto Mexicano de Tecnología del Agua Paseo Cuauhnáhuac 8532, Jiutepec, Morelos, México. C.P Fax (52) Abstract. The possibility of removing soluble arsenic from water during the application of a treatment for the removal of iron and manganese contained in that same water, has been reported by several authors. In spite of this, it is not yet possible to know in a precise manner the efficiency of removal to be accomplished in a specific case, which is why it is necessary to perform studies on a small scale with this objective. This work presents the results obtained in a pilot study performed on water from a group of wells that supply water to an important city in Mexico. The technology chosen for water treatment was the socalled natural zeolite adsorption-oxidation, which efficiently removes iron and manganese from water. In previous laboratory studies, it was also possible to prove the removal of arsenic through sorption, on this type of material. Most of the tests were performed with the mixture of water from several wells. Arsenic concentration varied between.21 and.32 mg/l, iron between.41 and 3.18 mg/l, and manganese between 1.1 and 1.3 mg/l. Total arsenic removal was between 19 and 6 %. The results show that the achieved arsenic removal is the sum of arsenic co-precipitation with iron, and the arsenic adsorption of manganese oxides that cover the zeolite. For a clean bed, arsenic adsorption of zeolite seems to be the main mechanism. However, as bed operation time passes, the sorbent material reduces its removal efficiency, leaving the most important role to co-precipitation. Tests were performed on two different water filtration velocities through the bed: 7.6 and 14 m3/m2h, the low velocity produced higher efficiencies, although the difference is not highly significant Key words: Arsenic, water treatment, adsorption, coprecipitation, zeolite, manganese oxides. Introduction. Up until a few years ago, the problem of arsenic contamination of drinking water supplies in México mainly occurred in areas such as the Comarca Lagunera region (located in part of the states of Coahuila and Durango), Chihuahua in northern México, and Zimapan, Hidalgo, in central México. However, recently, the existence of drinking water supplies that exceed the concentration limit established by Mexican standards (.3 mg As/L) has been discovered in locations in four other states in the Mexican Republic (Baja California Sur, Guanajuato, Sinaloa and Morelos). Some of these 1 water supplies are being partially treated with reverse-osmosis systems, producing arsenicfree water, which is exclusively used for consumption. This process is highly effective for arsenic removal, but the initial investment and maintenance costs are inconveniently high. Chronic arsenic exposure through drinking water causes characteristic skin lesions such as pigmentation changes, hyperkeratosis palmoplantaris, and papular hyperkeratosis, as well as epidermoid carcinoma. Alterations characterized by cyanosis and progressive loss of circulation in the hands and feet, which may lead to Raynaud s disease (gangrene type) and, consequently, to amputations, have been presented. There is also written information regarding conjunctivitis, myocarditis, vascular problems, and irritation of the respiratory apparatus and the skin 1. Due to these problems, the Official Mexican Standard NOM 127-SSA regarding the maximum allowable limit for arsenic concentration in drinking water has been modified, programming an annual decrease of.5 mg/l, which began with.5 mg/l in the year 2, reaching.25 mg/l in 25. Faced with this situation, it is necessary to invest resources and effort into analyzing the available arsenic removal technologies and researching alternative technologies. A very important factor in deciding which treatment is the most appropriate for arsenic removal is knowing what other elements or chemical compounds are present in the same water supply. The possibility of removing soluble arsenic from water during the application of a treatment for the removal of iron and manganese contained in the same water has been reported by several authors 3,4,5. In spite of this, it is not yet possible to know in a precise manner the efficiency of removal that could be accomplished in a specific case, therefore it is needed to perform studies on a small scale with this objective. This work presents the results obtained in a pilot study performed on water from a group of wells that supply water to an important city in México. The technology chosen for the water treatment is called adsorption-oxidation on natural zeolite, which efficiently removes iron and manganese from water. The process of removing iron and manganese basically consists of the adsorption, and subsequent continuous oxidation of these elements on natural zeolite (clinoptilolite type) coated with manganese oxide. Zeolites are complex natural aluminosilicates found in certain volcanic rocks. Zeolite of the clinoptilolite type has the following formula: Na 6 ((AlO 2 ) 6 (SiO 2 ) 3 )) 24 H 2 O, in which sodium is the main interchangeable element, although, it can also contain other cations such as potassium and calcium, in a smaller proportion. During the initial stage of the process, manganese from water is introduced into the zeolite through ion exchange. Once the zeolite is saturated, an oxidation process is performed with the continuous application of chlorine, gradually forming a coat of manganese oxide, which adsorbs iron and manganese dissolved in the water 6. Previous laboratory studies were also able to prove the removal of arsenic through sorption on this type of material. 2 The objectives of this study are: 1) to differentiate the removal mechanisms that intervene in the process, 2) to know the importance of the rate of filtration through zeolite beds in the process s efficiency, and 3) to determine the technical feasibility of applying this process to the water under study, in such as way that will allow obtaining arsenic concentrations that will comply with the.25 mg/l maximum allowable limit, established by Mexican standards for the year 25. Methodology. The pilot study was performed whit two types of water. The first (Type I) was a mixture of water extracted from several wells and conducted through 3-inch diameter steel pipes. Because it is mixture, the characteristics of water transported through the pipe vary according to which wells are operating at the time. During the test, water characteristics were: arsenic.22 to.29 mg/l; manganese 1.1 to 1.3 mg/l; iron.41 to.65 mg/l with increases for short periods of time of up to 1.7 to 3.18 mg/l; CO to 41.7 mg/l; ph 6.5 to 7.3; electric conductivity.446 to.5 ms/cm, and temperature 22 to 28 Celsius. The other water type (Type II) came from one well, and the water characteristics were: arsenic.38 to.43 mg/l; manganese 2.6 to 2.8 mg/l; iron 1.8 to 1.17 mg/l; ph = 7.48 and conductivity =.759 ms/cm. The zeolite used for the study, coated with manganese oxide, was obtained from one of the manganese removal plants that operate with the adsorption-oxidation process on natural zeolite. Tests on water Type I. Two treatment s phases were performed on this water type. The first had the objective of observing the removal mechanisms separately, meaning a) iron and arsenic coprecipitation, followed by arsenic adsorption on zeolite and b) both mechanisms performed at the same time. In the first case, the treatment train consisted of the following stages: aeration, sand filtration, chlorination, and filtration on zeolite coated with manganese oxide. Aeration was performed with two objectives: to desorb most of the carbon dioxide (CO 2 ) from the water and oxidizing the iron in order to form precipitates. Previous experiences with water containing similar concentrations of this gas show that it is necessary to remove it before passing water through filters containing granular material 7. In order to avoid its accumulation in the filter s bed, which causes shorter filtration life and a less efficient process. Desorption of carbon dioxide was performed in a model of aeration trays. The model consists of a distribution box, five aeration trays separated from each other by 3 cm, and a collection box for the treated water. A wire mesh was placed at the bottom of each tray and an 8-cm bed of crushed volcanic rock on top, in order to distribute the flow and increase the efficiency of the desorption process. The previously aerated water was transported through a peristaltic pump into a 15-cm diameter filtration column built with acrylic and filled with 34-cm thick sand (sand filter). 3 Sand size was.4 to.7 mm in diameter. The objective of this stage was to separate solid iron precipitates. The flow rate treated by this filter was 2.3 L/min. The objective of the following stage - filtration through a bed of zeolite - was to remove the remaining manganese and arsenic. With the objective of evaluating the effect that the rate of filtration has on the efficiency of arsenic removal, two filtration columns were used, operating in parallel, one at a rate of 7.6 and the other at 14.1 m 3 /m 2 h. These columns were 9-cm in diameter and 56-cm in bed thickness. The size s zeolite coated with manganese oxide used for this study was.42 to 1.4 mm with equivalent diameter.89 millimeters. With the objective of continuously regenerating the manganese oxide coat from the granular material, sodium hypochlorite added to the sand filter effluent before dividing the flow that was introduced into the zeolite-filled columns. Free residual chlorine in the effluent of the zeolite filters remained between.5 and 1.4 mg/l. The second phase of experimentation with water Type I consisted of all the stages of the previously described treatment train, without sand filtration. Tests on water Type II. Tests with this type of water were performed with the objective of evaluating the effect of regenerating zeolite coated with manganese oxide using potassium permanganate. The treatment train consisted of aeration, chlorination and adsorption on zeolite coated with manganese oxide. The stage of adsorption (filtration through zeolite) was also performed in two parallel filters, both using a 4.8 m 3 /m 2 h filtration rate. Before beginning the tests, both beds were backwashed with clean water and one of them was placed in regeneration, immersed in.5% potassium permanganate (KMnO4) solution for approximately 3 hours. Parameters analyzed and analytical methods. During the test, samples of raw water were taken in order to determine the amount of iron, manganese, arsenic, carbon dioxide, ph and conductivity. After aeration, the concentration of carbon dioxide was measured. The effluent from the sand filter was analyzed periodically to determine the concentration of iron, manganese, arsenic, ph and conductivity. After adding chlorine and before inserting water in the filters packed with zeolite, a sample collected to determine concentration of free chlorine. Last, the water produced by the zeolite packed filters was sampled periodically to determine the concentration of iron, manganese, arsenic, chlorine, and ph. Determination of iron and manganese concentrations was performed through colorimetric methods using a portable spectrophotometer. Manganese concentration was determined using the PAN indicator 1-(2-Pyridylazo)-2-Naphthol, while total iron concentration was measured with the ferrozine 3-(2-pyridyl)-5, 6-bis(4-phenylsulfonic acid)-1,2,4-triazine, monosodium salt. The ph from the samples was measured using the ORION 42 ph meter. Conductivity and total dissolved solids were measured using the ORION 13 conductivity meter. 4 Samples for determination of arsenic were preserved with HNO 3 and transferred to the laboratory for analysis with hydride generation atomic absorption spectrometry. Photo 1. Pilot system: aeration trays, sand filter and zeolite filters. Results. The use of the aeration trays system allowed obtaining an average of 79% CO 2 removal, which avoided operation problems with the filters. Fe (mg/l) Aerador + Fitro de arena + Filtro de zeolita Aerador + Filtro de zeolita Efluente filtro arena Efluente Zeolita (7.6 m/h) Efluente Zeolita (14.1 m/h) Aeration also accomplished oxidation and, therefore, the formation of insoluble iron compounds that were efficiently retained by the sand filter, or by the zeolite filters, according to the case. Graphic 1 shows the concentration of iron in pre-treated water (raw water) and in effluents from the sand filter and the zeolite filters. A vertical line separates operation time of the treatment train, left side with sand filter and right side without it. Graphic 1. Iron removal, water Type I. 5 Mn (mg/l) Aerador + Fitro de arena + Filtro de zeolita Aerador + Filtro de zeolita Efluente arena Efluente Zeolita (7.6 m/h) Efluente Zeolita (14.1 m/h) Graphic 2 displays the evolution of manganese regarding operation time. Due to the fact that the rate of manganese oxidation through aeration is very slow, maximum 15% removal was obtained in the sand filter. However, dissolved manganese adsorption was carried out efficiently in the zeolite filters. The iron and manganese removal was not affected by the filtration rate Graphic 2. Manganese removal, water Type I. As mentioned previously, the original treatment process don t include the sand filter; the objective of using it was to differentiate the arsenic removal mechanisms in this process. The affinity between arsenic and iron oxides or hydroxides is well-known, and so this study confirmed that a fraction of arsenic was coprecipitated or adsorbed by iron compounds formed during oxidation, and retained in the sand filter. Another fraction of arsenic was removed through adsorption in the manganese oxides that coated the zeolite. As (mg/l) Aerador + Fitro de arena + Filtro de zeolita Aerador + Filtro de zeolita Efluente arena Efluente zeolita (7.6 m/h) Efluente zeolita (14.1 m/h) Graphic 3 shows the arsenic concentration curves regarding operation time, distinguishing the contribution of the sand filter and the zeolite filters in removing this element. Once the sand filter was taken out of the operation, the physical retention of iron-arsenic coprecipitates and arsenic adsorption took place in the zeolite bed Graphic 3. Evolution of arsenic, water Type I. The concentration of arsenic obtained in the zeolite filter, which works at a low rate (7.6 m/h) is slightly smaller than the concentration obtained when working with a 14.1 m/h rate. The difference is accentuated after approximately 6 hours of operation. It is important to mention that, during the study, the process was able to produce water with an arsenic 6 concentration of less than.25 mg/l, which is the maximum limit to be allowable in México for water destined to human consumption, starting in 25. Remoción de As (%) Efluente arena Efluente Zeolita (7.6 m/h) Efluente Zeolita(14.1 m/h) Graphic 4. Arsenic removal profiles As, (mg/l) Graphic 4 displays the percentages of arsenic removal in sand and in zeolite, regarding time. The total removal obtained during the process, in a certain amount of time, is the sum of removal in the sand plus removal in a zeolite filter. We can see that, at the beginning of the process, the largest percentage of arsenic is removed by adsorption in the zeolite bed and, as time passes, this percentage decreases. This reduction is due to the gradual saturation of arsenic adsorption sites in the zeolite s manganese oxide coat, despite of the continuous adding of chlorine, which guarantees the oxidation of the adsorbed manganese and, therefore, the formation of a fresh layer of manganese oxide. As (mg/l) Zeolita regenerada con cloro Zeolita regenerada con KMnO With the purpose of observing the effect of another oxidant instead of hypochlorite as a regenerator, potassium permanganate was used to regenerate the bed. Once the bed was regenerated, the system began operating with a hydraulic rate of 4.8 m 3 / m 2 h for both filters. During this test, chlorine addition was maintained in the feed water of both filters. The concentrations of arsenic obtained in the effluents of the filters are displayed in graphic 5. Graphic 5. Evolution of arsenic, water Type I. Potassium permanganate manages to more effectively regenerate the sites of arsenic adsorption in the bed of zeolite, which means a higher removal of the contaminant. At the beginning, removal efficiency is 77.5% in the bed regenerated with potassium permanganate, against 47.5% in the bed that was not treated with this reagent. However, the 7 graphic shows the increment in arsenic concentration in treated water, meaning, the gradual saturation of the adsorbent is noticeable. Conclusions. The results of this study allow concluding that the process of iron and manganese removal through adsorption and oxidation on natural zeolite is capable of removing arsenic when these three contaminants are present in the water. This arsenic removal process is accomplished by: 1) arsenic coprecipitation with insoluble iron compounds which are physically retained in the bed of zeolite and 2) arsenic adsorption in the manganese oxides that coat the zeolite. For the particular case of the water under study, up to 38% of arsenic is removed through coprecipitation and filtration, while 5 to 6% is retained by adsorption. The effect of the rate of filtration through the beds of zeolite does not represent a highly significant difference on the efficiency of arsenic removal, at least not with the two filtration rates used in this study (7.6 and 14.1 m 3 /m 2 h). During the system operation, the produced water always maintained an residual arsenic concentration of less than.25 mg/l, which will be the maximum allowable limit for water destined to human consumption, according to Mexican standards. The adsorption capacity of zeolite coated with manganese oxides decreases as operation time increases, in spite of the continuous adding of chlorine. With the objective of defining the possibility of using this system to remove arsenic on a real scale, it is necessary to perform complementary laboratory studies of the regeneration of the material with sodium hypochlorite and with potassium permanganate, and to determine if this adsorption capacity remains constant or if it diminishes as the saturationregeneration cycles to which the bed of zeolite has been subjected increase. References 1 Cortés Muñoz, J.; Rivera Huerta, M. L.; Piña Soberanis, M.; Martín Domínguez, A.; Bedolla Vázquez, L. Evaluación de filtros intradomiciliarios, puesta en marcha de dos plantas potabilizadoras en Zimapán, Hidalgo y efectos a la salud asociados con la exposición a arsénico. Informe final. Instituto Mexicano de Tecnología del Agua. TC- 991, Modificación a la Norma Oficial Mexicana NOM-127-SSA1-1994, Salud Ambiental. Agua para uso y consumo humano. Límites permisibles de calidad y tratamiento a que debe someterse el agua para su potabilización. Diario Oficial de la Federación, 22 de noviembre de 2. 8 3 Viraraghavan, T; Subramanian, K. S.; Aruldoss, J. A. Arsenic in Drinking Water- Problems and Solutions. Wat. Sci. Tech. Vol. 4, No. 2 pp 69-76, Edwards, M. Chemistry of arsenic removal during coagulation and Fe-Mn oxidation. Journal of American Water Works Association, 86 (9), pp McNeill, L.S., and Edwards, M.A. Predicting As removal during metal hydroxide precipitation. Jour. AWWA, 89(1):75 6 Petkova Simeonova, V., M., Rivera Huerta, M. L., Piña Soberanis, M. Remoción de hierro y manganeso por adsorción en medios de contacto no convencionales. Informe final. Instituto Mexicano de Tecnología del Agua. TC-9531, Piña Soberanis, M., Rivera Huerta, M. L., Montellano Palacios, L., Vázquez Antonio, M.I..
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