Heavy metal removal mechanisms in constructed wetland wastewater treatment

Abstract

The detrimental effects of the untreated release of compounds containing heavy metals to the environment has been well documented. Numerous energy-intensive and/or labor intensive technologies have been developed to remove metals from various types of wastewater. However, constructed wetland (CW) treatment of wastewater has been shown to be an effective method for reducing or removing biochemical oxygen demand (BOD), nitrogen, phosphorus, and solids (both settleable and colloidal) through passive means which are much less resource-intensive. A limited amount of research has been conducted which indicates the removal of metals may be possible using CW systems. The primary objectives of this study were to confirm the mechanisms responsible for the removal of metallic compounds in the substrates (soils) of constructed wetlands, and to examine the effect different types of substrate materials have on the removal of metals from wastewaters such as acid mine drainage and landfill leachate -wastewaters which typically contain high levels of various metals. Mechanisms identified in the literature as being responsible for the removal of metals in the wetland environment include filtration, oxidation, precipitation, absorption, complexation, and plant uptake. A series of static Batch Absorption Experiments were conducted using the following pairs of absorbates and absorbents: copper/peat, copper/yard waste compost; copper/sand, iron/sand and iron/peat. Copper was observed to be readily absorbed by the organic soils (peat and yard waste compost), and generally followed the Freundlich Absorption Isotherm. The absorption of both copper and iron by sand was minimal, indicating the presence of organic material was important to the absorption process. A dynamic Column Experiment was conducted utilizing copper as the absorbate and peat as the absorbent. The removal of 95% of the applied copper was consistently achieved through the combination of absorption, complexation,and the filtration of precipitate.