ORTHOPHOSPHATE REMOVAL FROM SIMULATED AGRICULTURAL RUNOFF USING ZEROVALENT IRON ENHANCED SOIL MEDIA

Agricultural runoff often contains high orthophosphate (ortho-P) loads due to the accumulation of phosphorus (P) in soils. This accumulation can be caused by fertilizer application, manure and litter application, or animal waste where feed contained significant amounts of P. With increasing knowledge of the environmental impacts of ortho-P, efforts have been made to research effective treatment solutions for this runoff and minimize receiving water impairments. An emerging trend in this field has been the use of zerovalent iron (ZVI) as an adsorbent. ZVI has been shown to improve the removal of dissolved ortho-P from stormwater over traditional filtration and sedimentation systems. The removal of ortho-P from simulated runoff using ZVI-amended media was investigated in this study. The main objective of this research was to gather data to better understand and predict the lifespan of ZVI when treating high concentrations of ortho-P. Column tests were performed with varying depths and percentages of sand, loam, and ZVI composing the media. Sand and loam percentages were roughly 70% and 30%, respectively, while ZVI percentages tested were 0.5%, 1%, and 3%. These media mixes were tested at 6-inch and 12-inch media depths. The columns were subjected to target influent ortho-P concentrations of 4.57 mg-P/L over decades of simulated runoff. After roughly nine years of simulated runoff, all columns except for the column containing 3% ZVI and 12-inch media depth had lost their adsorption capacities. This last column lost its adsorption capacity around year twenty. The results indicated a strong correlation between ZVI mass in the columns and adsorption capacity. Columns with more ZVI mass correlated to higher cumulative ortho-P mass removed and longer ZVI lifespans. Though increased media depth also corresponded to better removal percentages, it was determined that this increased performance was likely due to the higher ZVI mass in the columns. Time-series percent ortho-P removal results for each column were used in the creation of a tool for estimating ZVI lifespan in field applications. This tool employed the Langmuir isotherm as the basis for predicting adsorption over time for the ZVI-amended media. Removal data from each column was used in developing this model. This tool allows the user to input values for the expected yearly volume of runoff at a site, the expected average ortho-P concentration, and the desired media depth and ZVI percentage. Using these inputs, the tool produces a predictive percent removal curve over a number of years that can estimate when the ZVI’s useful life will end, and therefore, when actual system maintenance (ZVI addition) is necessary