Removal of arsenic and zinc using different laboratory model wetland systems

Industrial and mining activities are an increasing threat to natural sites like wetlands and ponds because of the pollution by heavy metals and particularly arsenic (As) which they create. Four different laboratory scale model wetland systems, simulating a subsurface water wetland (SWW), free water surface wetland (FWSW), hydroponic system (HP), and an algae pond (AP) were initially loaded with water containing 5 mg/L of Zn and 0.5 mg/L of As as the main contaminants. The experiments run discontinuously and water losses by evapotranspiration were compensated periodically by distilled water. SWW, FWSW and HP were planted with Juncus effusus. The aim of this investigation was above all to study the removal of As in anthropogenically influenced stagnant wetland systems. The AP system showed almost no changes in all parameters measured. In addition, for the HP system no depth gradients of the parameters could be observed. Nevertheless, the total concentrations decreased slightly over 90 days by about 25 % for As and about 30 % for Zn. Within the gravel bed systems (SWW and FWSW) As and Zn were completely removed from the water, whereas for both parameters the removal process in the SWW was considerably faster. In both gravel bed systems the changes in the iron concentrations and the redox potentials were completely different. During periods of comparatively low redox potential, the iron concentration of the pore water increased from 0.1 mg/L up to 3.0 mg/L for the FSW and to 6.8 mg/L for the SWW. In periods of a higher redox potential the iron concentration decreased. The utmost As removal from the water was found in the SWW. It was noted that this could not be explained by either the adsorption on the gravel or by the plant uptake alone. It can be assumed that by the combination of both effects within one system soil bound crystalline iron, which has a low As binding capacity, is dissolved and can function as a co-precipitation agent for As in oxic zones such as possibly on the rhizoplane of helophytes.