Abstract

Wetlands have been known as an efficient and low-cost technology in treating wastewater. It was reported that wetlands (natural and constructed) were used to purify (a) municipal; (b) industrial; (c) acidic; and (d) agricultural wastewater. Mercury has been used intensively by wide variety of industries because of its unique properties. Consequently, it was present at various levels in wastewater discharged. The ability of constructed wetlands to serve as a sink of mercury before entering larger aquatic systems has not been investigated. This study focuses on the capability of constructed wetlands (Free Water Surface) to remove mercury from contaminated water/wastewater and reduce the widespread dispersion of the toxic substance to streams, rivers, reservoirs, and oceans. Mercury removal pathways considered in the study are sorption to the solid phase and uptake by plants. Current design approaches of constructed wetlands lack essential parameters necessary to optimise their capabilities for mercury removal. To evaluate these parameters a combination of experimental and numerical modeling approaches were undertaken. Experimental investigations were conducted to investigate the ability of Water Hyacinth ( Eichhornia crassipes )--floating plants--and Reeds ( Phragmites communis )--rooted plants--to remove inorganic mercury compounds from water. Kinetics of mercury removal by floating and rooted plants in the concentration range of 5 to 150 ppb were investigated and the apparent removal rate coefficients were determined for a first order kinetic model. Numerical modeling investigations were then carried out to: (a) expand the results obtained in the experimental phase by examining the distribution of mercury forms in water including: HgOH +1 , Hg(OH) 3 -1 , Hg +2 , H9Cl +1 , HgCl 2 (aq), HgCl 3 -1 , HgCl 4 -2 , HgClOH (aq) and evaluating the amount of bioavailable mercury for plant uptake; (b) verify the effects of pH, temperature, and chloride concentrations in water on Hg speciation; (c) assess characteristics of mercury adsorption on four different sediment materials (kaolinite, natural soil, glokonite, and bentonite) at three pH values and three different adsorbent medium for five initial mercury concentrations (1E-7 to 1E-3 moles) using a triple layer adsorption model. The major results suggest that--properly designed--constructed wetlands can remove up to 95% of mercury discharged within short period of 3 days. The removal process occurs at in two phases fast and around 10 times slower thereafter. The uptake of the bioavailable form of mercury (He 2 +2 ) was influenced mainly by the pH of the water discharged. Higher adsorbent mass concentration in sediments and higher pH result in less amount of bioavailable mercury. The amount of bioavailable mercury was proportional to the initial mercury concentrations. It was concluded also that mercury has high affinity to be sorbed on all tested sediment mediums. The findings of this research could be also applied in preparing environmental risk impact assessments and could be considered as the first nuclei of establishing regulations (codes) governing the design of constructed wetlands to remove mercury from contaminated waters. However, a pilot field study is still necessary to validate the experimental results.