Remediation of Arsenic in Hawaii Soils – Laboratory and Field Pilot Studies of Geochemical Fixation
Hawaii Department of Health
10/01/06 – 09/30/07
Arsenic (As) has been known to cause a variety of adverse health effects, including skin and several internal cancers, and cardiovascular and neurological effects (National Research Council, 1999). Thus, exposure to As is a serious concern. Alarmingly, most Hawaii soils contain rather high levels of As. A recent survey of 22 soils of 4 orders (Andisols, Inceptisols, Oxisols, and Ultisols) around the State yielded an average of 50 mg/kg of total As; for comparison, an analysis of 24 soils from Mali, West Africa, yielded only 4 mg/kg total As with a range of 1.0 to 6.0 mg/kg (Hue, unpublished data). The probable cause of observed As levels in Hawaii soils is due to both basaltic parent rock origin and, in certain locations, historic use of As in agricultural pesticides (Hansen, 1962).
Former sugarcane fields in the Keaau area of the Island of Hawaii (“Big Island”) exhibit As levels ranging from 50 to 900 mg/kg. At the higher As soil concentrations, remedial measures may be necessary to reduce or eliminate potential human exposure health risks. The fraction of total soil As that is available for uptake in the human digestive system is referred to as the bioaccessible As fraction. The Hawaii Department of Health (HDOH) has determined that the potential public health risks from soil As are from the bioaccessible fraction, and therefore remedial technologies to reduce As bioaccessibility are the focus of the subject research (HDOH 2006).
Remedial technologies can be divided into 2 broad categories based on the implementation strategy: In situ remediation refers to treatment of contaminants in place within the environmental media, whereas ex situ remediation involves physical removal of the media and treatment of contaminants within an engineered setting (Pierzynski et al., 2005). Due to the high cost of removal, transportation and disposal of soil contaminants (the most common ex situ remedy), in-situ remedies are preferred whenever technically feasible and effective. The authors believe that geochemical fixation and phytoremediation are potentially feasible and cost-effective treatments of As contaminated soils (Cutler, 2005; US-EPA, 2002; Terry and Banuelos, 1999).
Geochemical fixation of heavy metals in soils is a well-developed technology. Arsenicimpacted soils have been successfully treated by chemical fixation and stabilization/solidification at a number of sites. Cutler (2005) has shown that iron hydroxide amendments present a potential remedial technology to reduce bioaccessible As in Hawaii soils. Other potential As fixation amendments are manganese and aluminum oxide materials.
The primary objective of the study was to develop potential cost-effective remedial technologies to reduce bioaccessible arsenic levels in Hawaii soils. Soils from the Keaau area of the Big Island were of primary focus, since they are known to exhibit elevated As levels above background concentrations. Specific objectives included:
- 1. Developing a better understanding of the geochemistry of As in soils from Kea’au, Hawaii, including its aqueous and solid phase (mineralogical) associations, and sorption, mobility and bioaccessibility (leaching) properties;
2. Measuring the performance of various soil amendments on the reduction of bioaccessible As in Keaau soils;
3. Supporting HDOH in the public’s awareness of As risk, mitigation and remediation alternatives.
In conjunction with Dr. Nguyen Hue, a series of laboratory tests were conducted to determine the sorption and desorption (leaching) properties of the Keaau soils to arsenic. In vitro arsenic bioaccessibility testing to date (ERM 2005) indicated that the soils have an extraordinary capacity to adsorb and retain arsenic, and limit its availability for uptake in the digestive system and leachability to soil solutions. The researchers quantified the sorption/desorption properties of the Keaau soils, to include the development of sorption/desorption isotherms.
The locations of arsenic within the soil material was defined by performing a series of “sequential extractions,” modified from protocols developed by using a series of progressively more robust extraction fluids (using methods adapted from Keon et al. 2001). These tests better defined the way that arsenic is bound in the Kea’au soils, and aid in evaluating both risk assessments and potential remedial solutions.
Dr. Chittaranjan Ray
Water Resources Research Center
University of Hawaii at Manoa