This research deals with removal of As(V) from waters containing different levels of Ca²⁺, alkalinity, and silicate using Arsenil™ - a nanophase Fe-Mn oxide coated natural zeolite (U.S. Patent No. 6,790,363). Studies have shown that adsorptive media perform poorly under the following water chemistries: high pH; high Ca²⁺ and alkalinity; and, high silicate. In high Ca²⁺ and alkalinity waters, the primary reason may be due to the CaCO₃ formation resulting in surface passivation/fouling and/or site inaccessibility. Lowering the pH of these waters to 7 improved the As(V) removal efficiency of the medium, and this may be attributed to the reduction or elimination of water alkalinity. In the first part of the study, Dallas, Texas municipal water was spiked with As(V) only as it contains low Ca²⁺, alkalinity, and silicate levels. This is representative of As water chemistry in the Northeast and Midwest regions of the USA. The Dallas water was spiked with 25 ppm Si along with As(V) to simulate water conditions in the Pacific Southwest regions. The influent water pH was maintained at 7 as these waters contain high alkalinity and in the presence of high Ca²⁺ forms CaCO₃ in the intra- and inter-particle void spaces resulting in decreased As sorption. Based on thermodynamics, the dominant silicate species in the waters with pH ranging from 6 to 8.5 is H₄SiO₄ ⁰ (a neutral species), and lowering the pH will not revert significantly to charged silicate species. Since the pH was decreased to 7 in this study, no attempt was made to increase water alkalinity. First, bench scale studies were conducted followed by pilot-scale studies using As(V) species, as these are the most dominant oxidation state in US waters. Bench scale studies were useful to provide As sorption properties and Empty Bed Contact Time (EBCT) for designing the column studies. The column studies were performed using Dallas municipal water containing pH 8.2, low alkalinity and low Ca²⁺ and Si. In both the columns the effluent As levels were below the detection limits. There was also no significant sorption of Ca²⁺ and Si by the Arsenil™ medium. These studies show that Arsenil™ can be used successfully to treat high pH, and low Ca and alkalinity water with no chemical pretreatment but for water with high Ca and alkalinity pretreatment is necessary. For high Si containing water but with low Ca and alkalinity, no pretreatment is recommended. The use of dual serial columns or roughing and polishing columns will significantly improve the As removal efficiency and EBCT. Includes 29 references, tables, figures.