Investigation of zinc removal capacities of different sorbent materials to be used in constructed wetlands

Abstract

There is an increasing demand for better water quality in order to safeguard public health, the social security and accomplish environmental integrity. It has been found over the past couple of years that health hazards associated with heavy metal have been on the rise, particularly the chronic health problems due to the ingestion or consumption of even low doses of heavy metal-rich waters. Accumulation of such metals is reported mainly due to non-treatment or poor treatment of industrial wastewaters. Lack of tertiary treatment may have attributed to this growing problem and hence the environmental pollution. Constructed wetlands have therefore received great attention in the recent past as a tertiary treatment method or a polishing technique due to low construction and operation costs, minimum maintenance and also as an environmental friendly system. However, finding a low-cost sorbent material which can be used as an alternative to activated carbon has been a problem for decades in wastewater treatment industry, especially in developing countries. Therefore, the present study focuses on the applicability of low-cost sorbent materials that can be used in constructed wetlands as a filter medium. The focus was on four sorbent materials: tile, brick, saw dust and rice husks, which were selected based on their local availability. Laboratory-scale experiments were performed to investigate their maximum adsorption capacity and removal efficiency with a synthetic Zinc solution. The Results revealed that tile material has the highest adsorption capacity (47.6 mg/g) and removal efficiency, (98%) while brick (37.0 mg/g, 86%), sawdust (20.4 mg/g, 80%) and rice husks (15.8 mg/g, 64%) have relatively low adsorption capacities and removal efficiencies, respectively. The percentage removal of Zinc by all the four sorbent materials increased with an increase of contact time. The kinetics of adsorption were relatively fast for all tested low-cost materials. The equilibrium data were correlated with both Langmuir and Freundlich isotherms. Adsorption isotherms are well described by Langmuir isotherms. The separation factor of equilibrium (RL) indicates favourable isotherms (0< RL >1) for all tested materials. Characterization of four sorbent materials was done by undertaking SEM, XRD and FTIR analyses. It can be concluded from the results that, the low-cost sorbent materials that were tested can be an attractive substitute for activated carbon in removing Zn from industrial wastewaters. Keywords: adsorption isotherms, constructed wetlands, sorbent material, Zinc