The effect of substrate media on the removal of arsenic, boron and iron from an acidic wastewater in planted column reactors

Abstract

Acidic wastewaters, such as occur in acid mine drainage, typically contain heavy metals and metalloids that pose a serious threat to receiving waters, due to their high toxicity. In this study, vertical flow wetland columns, using a range of filter media, were investigated for their potential to provide an effective onsite treatment for acidic wastewaters with a pH value of 2.0 ± 0.1. The effectiveness of four types of wetland media: gravel, cocopeat, zeolite and limestone, was studied for the removal of arsenic, boron and iron, under an average hydraulic loading of 0.073 m3/m2d. On average, limestone wetland columns gave the highest removal percentage for arsenic (99%) and iron (98%), followed by zeolite columns (92% removal for arsenic and 86% for iron). Although gravel columns were able to remove 43% of dissolved arsenic (from average input of 3.0 ± 0.1 mg/L), they were not able to remove iron simultaneously. In contrast, wetland columns with cocopeat media only showed modest capacity for arsenic removal (9%), but greater capacity for removing iron (46%), and cocopeat was the only wetland media that demonstrated potential to remove boron. Overall, the results indicated that the most effective mechanism of arsenic removal in vertical flow wetlands is coprecipitation with iron, which can be enhanced by using alkaline wetland media to increase the pH of the wastewater. Combinations of media appear worthy of examination in order to optimise the efficacy and sustainability of heavy metal removal.