Arsenic Removal from Drinking Waters Containing Varying Alkalinityand Silicate Levels

摘要

This research deals with removal of As(V) from waters containing different levels ofrnCa2+, alkalinity, and silicate using ArsenilTM - a nanophase Fe-Mn oxide coated natural zeolitern(U.S. Patent No. 6,790,363). Studies have shown that adsorptive media perform poorly underrnfollowing water chemistries: 1) high pH, 2) high Ca2+ and alkalinity, and 3) high silicate. In highrnCa2+ and alkalinity waters, the primary reason may be due to the CaCO3 formation resulting inrnsurface passivation/fouling and/or site inaccessibility. Lowering the pH of these waters to 7rnimproved the As(V) removal efficiency of the medium, and this may be attributed to thernreduction or elimination of water alkalinity (See Appendix I).rnIn the first part of the study, Dallas, TX municipal water was spiked with As(V) only as itrncontains low Ca2+, alkalinity, and silicate levels. This is representative of As water chemistry inrnNortheast and Midwest regions of USA. The Dallas water spiked with 25 ppm Si along withrnAs(V) simulate water conditions in the Pacific Southwest regions. We maintained the influentrnwater pH at 7 as these waters contain high alkalinity and in the presence of high Ca2+ formsrnCaCO3 in the intra- and inter-particle void spaces resulting in decreased As sorption. Based onrnthermodynamics, the dominant silicate species in the waters with pH ranging from 6 to 8.5 isrnH4SiO40 ( a neutral species), and lowering the pH will not revert significantly to charged silicaternspecies. Since, the pH was decreased to 7 in this study no attempt was made to increase waterrnalkalinity.rnFirst, bench scale studies were conducted followed by pilot-scale studies using As(V)rnspecies, as these are the most dominant oxidation state in US waters. Bench scale studies werernuseful to provide As sorption properties and Empty Bed Contact Time (EBCT) for designing therncolumn studies.rnThe column studies were performed using Dallas, TX municipal water containing pH 8.2,rnlow alkalinity (76 mg CaCO3 per L) and low Ca2+ (40 ppm) and Si (<3 ppm). The influent waterrnspiked with 38 ppb As with EBCT of 5 min, and 115 ppb As with EBCT of 10 min passedrn10,138 and 5,242 bed volumes; respectively. In case of the column studies with influent waterrnmaintained at 25 ppm Si and pH 7, the column with As level of 25 ppb with EBCT of 10 minrnpassed 14,911 bed volumes and the 50 ppb As spiked column with EBCT of 10 min passedrn8,133 bed volumes. In both the columns the effluent As levels were below the detection limits.rnThere was also no significant sorption of Ca2+ and Si by the ArsenilTM medium.