An overview of arsenic removal by pressure-driven membrane processes

Management of hazardous wastes, such as arsenic, is one of major public concern. Arsenic is a naturally occurring metalloid, which is widely distributed in nature. Recently, arsenic in drinking water attracted attention because some of the drinking water resources contain considerable concentrations of arsenic which cause acute and chronic symptoms in many countries, especially in Bangladesh, China, Mongolia, and Taiwan. In 2001, the USEPA promulgated a rule lowering the arsenic MCL from 5 µgL−1 to 10 µgL−1. This paper offers an overview of geochemistry, distribution, sources, regulations, acute and chronic symptoms, and applications of membrane technologies in the water treatment research that have already been realized or that are suggested on the basis of bench or lab scale research. These membrane technologies include RO, NF, UF, and MF. Most of theses applications have proven to be reliable in removing arsenic from water. The possible influence of some source water parameters, membrane material, membrane types, membrane processes on arsenic removal efficiency by membrane technologies are also explored. This review paper also offers data relating to regulations of arsenic standard, acute and chronic symptoms that are caused by the exposure of arsenic to explain why water treatments need to use the membrane technology to meet the MCL standard.

RO generally removes particles larger than 0.0001 µ, whereas NF removes particles larger than 0.001 µ. UF is capable of separating contaminants with particle diameter as small as 0.01 µ while MF only can reject contaminants with a diameter of 0.1 µ. RO and NF membrane processes are likely to effectively treat dissolved arsenic compared to UF and MF membrane processes, since arsenic species dissolved in water tend to have relatively low molecular weights [76]. The operation conditions, such as membrane material, water source, and pH value of solution, also affect the arsenic removal efficiency. Overall, RO and NF membrane pro- cesses have an excellent removal efficiency of arsenic; especially RO which can obtain over 95% arsenic removal efficiency. However, the percentage of product water that can be produced from the feed water for RO and NF is typically lower than the percentage of product water for UF and MF. The small pore size makes RO and NF membranes consume more energy to push water past the membrane than UF and MF membranes and are more prone to fouling than UF and MF membranes. The cost of membranes depends on membrane type, size and the degree of automation of the design. RO and NF membrane processes are normally more expensive than UF and MF membrane processes. The membrane technologies, RO, NF, UF, and MF have been demonstrated to be sufficiently effective to remove arsenic from water and meet the arsenic MCL standard. However, the effectiveness of membrane technologies is sensitive to a variety of source water characteristics, water contaminants, arsenic species, and membrane characteristics. In fact, the wide range of water recovery applications comprises a variety of fluid environments, some of which may be detrimental to certain polymeric materials and the morphological characteristics of the membrane. Operating conditions such as temperature, pressure, solution pH and chemical compatibility must therefore always be considered. The operating costs also need to be taken into consideration. Different membrane materials, type, processes and operating conditions will affect the operating cost. No one-membrane material, membrane type and membrane process can be used in all the possible environments and requirements of different arsenic removal applications. Therefore, different membrane materials type and membrane processes should be evaluated to select the optimum for each situation. This review article offers some data about relationships between arsenic removal efficiency and different operating conditions by using different membrane material, types and processes. In order to increase the arsenic removal efficiency or de- crease operating costs, some water treatment processes were combined into the membrane technology. MF has been used in conjunction with precipitation/co-precipitation to remove solids containing arsenic. The efficiency of arsenic removal by using low-pressure driven membrane is highly dependent on the size distribution of arsenic-bearing particles in the source water. If arsenic is present in the particulate forms, membranes of relatively large pre-size may be effective for arsenic removal. The larger the arsenic-bearing particles the lower the numbers of arsenic that are able to pass through the given pore size membrane. The use of flocculants or coagulants will be able to increase the number and the size of arsenicbearing particles. A small floc is formed when the flocculants or coagulants are added. Considering the developing countries’ situation, such as low annual income and low electric popularization, traditional RO and NF membrane technology seems difficult to be applied due to its high-energy consumption. Therefore, the combination of a bicycle pump with tight RO membrane is investigated. On the basis of the results of literature, the removal efficiency for As V is reportedly much better than for As III by using membrane processes. Therefore, the use of an oxidizing agent, such as chlorine, is necessary for the improvement of higher arsenic removal rate if arsenic in the source water is primarily speculated as As III. However, oxidation is not an easy way to improve the efficiency since oxidant could damage the membrane. Some researchers found that some microorganisms can transform arsenate to arsenite without the addition of oxidant. It will be a possible way to improve the efficiency of arsenite removal by combining the membrane processes and biooxidation. In recent years, a tremendous amount of research has been conducted to identify technologies for arsenic removal that can be applied in rural areas. Membrane technologies could be the best choice for all these advantages to be used in point-of-use (POU) filters or household filters to help the drinking water meet the arsenic MCL in rural areas or some arsenic contaminated areas.