Arsenic Removal Using Nanoscale Magnetite: Lab Scale to Pilot Scale

Silica-coated magnetite nanoparticle (16 nm core; 34 nm shell).

Access to clean drinking water is a challenge for most of the world’s population; in particular, the reduction of highly toxic contaminants, such as arsenic and many pesticides is a major goal. (WorldHealthOrganization 2001) Arsenic from both natural and manmade sources is a common contaminant in water; recent data makes clear the correlation between its consumption and an increased level of bladder and rectal cancers.(Smedley and Kinniburgh 2002; U.S. EPA 2002) The U.S. has lowered the drinking water standard for arsenic to 10 ppb, a value that is challenging to reach in most water treatment plants. Some reports suggest that for public health concerns the standard should be moved even lower; as captured in the WHO 1999 report on water quality “…the more we learn about the human health effects of heavy metals the greater the concern they appear to be”.(Smedley and Kinniburgh 2002; U.S. EPA 2002) Worldwide, arsenic poisoning is a huge public health disaster in many developing nations, such as Bangladesh, where it has affected over 10-20% of the population.(Frisbie, Ortega et al. 2002; Lin, Tang et al. 2002) It also has remained a persistent contaminant here in the U.S. and a large fraction of the water samples in various parts of America will need to be treated for arsenic removal.(May, Wiedmeyer et al. 2001; Brown and Ross 2002; Karagas, Stukel et al. 2002; Magalhaes 2002; USGS 2002).

In response to its dangers, and its widespread occurrence globally, the problem of arsenic removal from water has attracted significant attention in the technical research community. Arsenic in water, in either the As(III) or As(V) state, exists as oxoanions which are large and polarizable; many of the conventional removal strategies for anion removal in waste water are best suited for hard ions and not easily adapted for arsenic.(Zheng, Goessler et al. 1998; Viraraghavan, Subramanian et al. 1999) More novel methods to directly address the arsenic problem have recently been proposed. These include the use of plants such as ferns to sequester arsenic from soils; muds and sands as materials for arsenic removal columns(Cheng, Van Geen et al. 2004); zero-valent iron as an in-situ treatment for arsenic(Kanel, Manning et al. 2005); and most relevant for this work, the use of mine tailings, which are predominantly iron oxide, as sorbents for treating contaminated sites. From this work, several important features of arsenic treatment systems emerge, including the disposal burden (gm As/kg sorbent) as well as the percent efficiency of As(III) and As(V) removal at various equilibrium arsenic concentrations. While each of these methods offers unique advantages, in particular the use of bulk sorbent materials requires the disposal of large masses of contaminated sorbent material loaded with relatively little arsenic and the use of ferric chloride fouls membranes and leaves large iron residuals in the water.

This project, coordinated by Tomson, Colvin and Li, has made considerable progress this year.  The focus of the past year’s research is aimed at (1) modeling the kinetic and hysteretic adsorption/desorption of arsenic by nanomagnetite with a multi-reaction model; (2) enhanced adsorption of arsenic in the presence of Zn2+ and (3) scale-up of packed column  treatment using nanoscale-magnetite in preparation for an on site, pilot scale demonstration in Guanajuato, Mexico.

Participating Researchers:

• Vicki Colvin
• Qilin Li
• Mason Tomson