(119b) Impact of Water Quality and Assessment of Adsorption Technologies for Arsenic Removal

The sources of arsenic in drinking water are both natural and anthropogenic.  In order to protect public health, the U.S. EPA, World Health Organization (WHO), and the European Commission have decided to lower the MCL of arsenic in drinking water to < 10 µg/L.  This stringent arsenic standard will inevitably require many utilities to upgrade their present systems or consider new treatment options.  Extensive literature and vendor surveys revealed that a large number of commercial and experimental adsorbents are available for arsenic removal.  Some of the commercially available materials are matured products (e.g., activated alumina) that have been widely tested, revealing both their attributes and limitations, while others are more recent products.  Results from several investigators (DeMarco et al., 2003; Greenleaf and SenGupta, 2005; Jekel et al., 1999; Pena et al., 2005) indicate that some of these adsorbents are very promising.

Bench-scale studies were conducted to develop a comparative assessment of several commercially available adsorbents for arsenic removal, and to assess their performance in the presence of phosphate, vanadium, and silica.  The primary objective of this work was to identify the impacts of water quality, and to define adsorbent properties that will minimize the effects of water quality and/or maximize arsenic adsorption capacity.  Additionally, the effect of temperature on the adsorption kinetics and capacities were investigated.  Iron modified activated alumina (AA-FS50), granular ferric hydroxide (GFH), granular ferric oxide (GFO), and a titanium-based media (MetSorb G) were used as the adsorbent.  In order to better understand the adsorption mechanics of the selected adsorbents, each was characterized according to its important physical/chemical properties, including particle size, bulk density, pH of zero-point charge (pH_zpc), surface area, and mineralogy. The concentrations of dissolved arsenic and vanadium were analyzed using Inductively Coupled Plasma – Mass Spectroscopy (ICP-MS); Ion Chromatography (IC) was used to analyze phosphorus.  The concentrations of molybdate-reactive (monomeric) silica in the samples were measured, and polymeric silica was estimated by the difference between the total and monomeric silica.

Results revealed that pH has a significant impact on the adsorption capacities. AA-FS50 showed a moderate decrease in adsorption capacity as pH increased, and phosphate had a small impact.  GFO media was only slightly affected by increased pH, but was significantly impacted by phosphate, and monomeric silica had some minor effects.  GFH performance decreased as pH increased, with phosphate and monomeric silica exhibiting some impact.  The adsorption performance of MetSorb G also decreased as pH increased, and phosphate and monomeric silica had some impact, although the interfering effects were less at a lower pH for this media.  The two iron oxide media and MetSorb G generally exhibited comparable capacities. For iron oxide based media, rate data are best described by a pseudo-first order kinetic model at each temperature and pH condition studied. At lower pH values, arsenic (V) exhibits greater removal rates than arsenic (III). An increase in temperature increases the overall adsorption reaction rate constant values for both arsenic (V) and arsenic (III). An examination of thermodynamic parameters reveals that adsorption of arsenic (III) as well as arsenic (V) by iron oxide media is an endothermic process and is spontaneous at the specific temperatures investigated.

References

DeMarco, M.J., SenGupta A.K., Greenleaf J.E., 2003. Arsenic removal using a polymeric/inorganic hybrid sorbent, Water Res. 37, 164-176.

Greenleaf, J. E. and SenGupta, A. K., 2005, Investigating New Arsenic Removal Technologies, Chapter 9.3.2, p. 524-544;  In Das, T. K., (ed.),  Toward Zero Discharge: Innovative Methodology and Technologies for Process Pollution Prevention, John Wiley and Sons, Hoboken, NJ.

 Jekel, M, and Seith, R., 1999. Comparison of conventional and new techniques for the removal of arsenic in a full-scale water treatment plant. Proceedings World Water Conference, Buenos Aires, Argentina.

Pena, M.E., Korfiatis, G.P., Patel, M., Lippincott, L., Meng, X., 2005. Adsorption of As (V) and As (III) by nanocrystalline titanium dioxide. Water Res. 39, 2327 – 2337.