Effect of migration of amorphous iron oxide on phosphorous spatial distribution in constructed wetland with horizontal sub-surface flow

In a laboratory-scale constructed wetland (CW) with horizontal sub-surface flow, the migration of amorphous oxide iron (AOI) in medium and spatial distribution of phosphorus (P) were researched. Five months after treating wastewater, AOI concentration of medium changed in different area of the wetland medium bed. Compared with the medium before wastewater treatment, it rose in front rhizosphere and decreased in whole non-rhizosphere, remained almost the same in the back rhizosphere. In addition, iron plaque containing much AOI appeared on surface of all plant roots. The Fe level of root iron plaque reached about 25.88 ± 9.92 (n = 6) and 17.33 ± 2.72 (n = 6) g Fe/kg dried root in the front and the back area respectively. These should be resulted from the migration of the AOI from non-rhizosphere to rhizosphere in the CW. The P distribution was impacted by the migration. The medium had higher P content in rhizosphere than the one in non-rhizosphere because of its higher AOI concentration. Corresponding to the Fe level of iron plaque in different areas, P levels of plant biomass and root iron plaque in front area also were higher than the ones in back area.

► Amorphous iron oxide (AIO) of medium changed inside the CW when treating wastewater.
► The medium had more AIO and P in rhizosphere than one in non-rhizosphere.
► Iron plaque appeared on surface of all plant roots.
► The P distribution was impacted by the migration of AIO.
► Iron plaque was largely higher P content than medium and plant biomass.