Friday, August 7, 2009

PS 93-177: Azolla caroliniana: A model for arsenic remediation

Alexander M. Duncan, Johan F. Gottgens, and Daryl F. Dwyer. University of Toledo


Arsenic contamination is widespread and has been linked to cancer in humans and to reduced biodiversity. Due to its extreme toxicity and high prevalence in the environment, the Department of Health and Human Services has ranked arsenic as the number one contaminant of concern. Conventional arsenic removal techniques rely heavily on the use of chemicals that both impact water quality and significantly raise operational costs. Wetlands have potential to function as inexpensive, efficient options for removing arsenic from contaminated water. As a first step to design a wetland for arsenic removal from wastewater, we determined the phytofiltration capacity of Azolla caroliniana. In a series of three randomized block design experiments, A. caroliniana was exposed to monomethylarsenic acid (MMAA), dimethylarsenic acid (DMAA), arsenite (As III) and arsenate (As V), tested for differences between adsorption to the plant and absorption into the plant, and exposed to different arsenate concentrations (0, 500, 1000, 1500 ppb) over 21 days. Arsenic and mineral levels in plant and solution samples were analyzed using ICP-OES. Resulting data were used to build a mass balance model designed to track arsenic movement among plant tissue, solution and by difference, air.


A. caroliniana absorbed arsenic from all treatments with tissue concentrations increasing from below detection level (9.5 mg kg-1) up to 530 mg kg-1 dry weight in four days.  Growth inhibition varied with toxicity following the trend DMAA < As(III) < MMAA < As(V). Adsorption of arsenic to the plant surface was not a significant portion of total detectable arsenic. A. caroliniana arsenic tissue concentration increased with increasing concentration of arsenate with no significant differences in growth (fresh weight). Significant differences in the concentration of Ca, K, P, Mg, Mn, Fe, Si, S, and Na within A. caroliniana biomass was evident among treatments. Mass balance modeling revealed a strong relationship (r2=0.899, 0.862, 0.947) between mass of arsenic removed and time in exposures of 500, 1000 and 1500 ppb, respectively. Extrapolation predicts an arsenic reduction to levels below 150 ppb (EPA’s recommendation for surface water) in 99-102 days, depending on initial concentration values. These outcomes provide managers a framework for designing wetlands for large scale remediation of arsenic waste.