Study of Suspended Solid in Constructed Wetland Using Rare Earth Elements

Abstract:

Constructed wetland (CW) is one of the commonly used technologies in wastewater treatment. By means of the biochemical interactions among water, microscopic organism, aquatic plant and sediments in natural environment CW can remove biochemical oxygen demand (BOD), ammoniacal nitrogen, suspended solid (SS) and heavy metals. In this study, rare earth elements (REEs) were used as a natural tracer for the study of SS in the CW.

The studied CW, Hebao Island free water surface CW, is located in Chiayi County, south Taiwan. The CW is designed for removing SS and BOD due to the pollution from livestock farms in the upstream area. However, the removal of SS was not effective. In some cases, the SS concentration of inflow is even higher than that of outflow. That the sediments on the slope were flushed into the CW was considered as the main problem. After all the refinement, the issue has not improved yet. In the study, the water samples were filtered with 1.0µm filter paper. Then, part of water samples were digested by ultrapure nitric acid to obtain the water representing the total of dissolved and suspended matters. The others were filtered by 0.1µm filter, which represent the matters in dissolved form. REEs and most of metals were subsequently measured with ICP-MS.

REEs generally have a unique source and would fractionate in certain regular patterns during biochemical reactions due to lanthanide contraction. They can be an excellent natural tracer in the environmental researches. After normalized by North American Shale Composite, the REEs pattern for the samples with the total of dissolved and suspended matters is characterized by a middle REE (MREE) enrichment and light REE (LREE) depletion. According to the previous theoretical studies, the MREE enrichment could be achieved by a selected adsorption of MREEs by organic matters, which is generally humic substance in natural surface water. It is suggested that the refinement of removal efficiency of SS should focus on organic matters but not sediments. On the contrary, the REEs pattern of dissolved matters is flat and shows no MREE enrichment, which means the major source of dissolved REEs is possibly sediments. In addition, a distinct europium positive anomaly can be observed. It reconfirms that the dissolution of feldspar in sediments was strongly involved.