Evaluating Steel Byproducts and Natural Minerals for Phosphate Adsorption from Agricultural Subsurface Drainage

The loss of phosphorus (P) from agricultural soils to surface waters is recognized as a key contributing factor to eutrophication in surface waters. Recent studies have shown that in addition to surface runoff, subsurface drainage can contribute substantially to the loss of P from soils. The use of adsorption materials to bind P has emerged as a promising technology for P removal. The objective of this study was to perform batch and column experiments to investigate the P adsorption potential of several materials for agricultural subsurface drainage treatment. Laboratory batch adsorption experiments were performed to determine the P adsorption capacities of natural minerals and industrial byproducts, including limestone, zeolite, calcite, steel slag, iron filings, and steel byproducts. Steel byproducts are a lowcost, readily available material that has high P adsorption potential. The steel byproducts included small chips, medium chips, and large turnings, which were collected from local machine shops. The impact of temperature, reaction time, pH, nitrate, sulfate, and dissolved organic carbon (DOC) on the adsorption of P were evaluated for this study. The results showed that iron-based industrial byproducts had adsorption capacities that are one order of magnitude higher than natural minerals. The three sizes of steel byproducts exhibited P adsorption capacities of 2.54 to 4.47 mg/g, which were comparable to the steel slag and the iron filings. The P adsorption capacity increased by factors of 1.2 to 2.8 when increasing temperatures from 5 to 30 °C for the selected materials. Steel byproducts exhibited fast P adsorption kinetics as more than 60% of the 24 h adsorption potential occurred within 8 h. Decreasing pH resulted in an increased adsorption capacity among steel byproducts. Nitrate and sulfate had little impact on P removal for steel byproducts while DOC inhibited P adsorption by up to 24%. Laboratory fixed-bed column adsorption experiments were performed to determine the P adsorption characteristics of steel byproducts and steel slag. The steel byproducts included small chips, medium chips, and large chips. The impact of initial P concentration, empty bed contact time (EBCT), pH, and DOC on the adsorption of P were evaluated for this study. The potential for P recovery by desorption and the ability to remove other contaminants was also investigated. The results showed that steel byproducts had cumulative P removals of 8.43 to 10.4 mg/g over 4800 empty bed volumes, while steel slag only removed 1.50 mg/g before exhaustion. Based on the results, medium steel chips were chosen for further testing. Higher initial P concentrations resulted in faster exhaustion and longer EBCTs increased P removal efficiencies and P adsorption capacities. Decreasing pH from 9.0 to 5.0 resulted in an increase in P adsorption by 56% for medium steel chips. DOC inhibited P adsorption by up to 38%. Recoveries of 59 to 83% of the attached P were measured using varying strengths of NaOH solution. Medium steel chips removed considerable amount of DOC, Cu, and Zn along with P in a mixed contaminant study. Overall, the results of this study suggest that steel byproducts are an efficient P adsorption material that can potentially be used for agricultural subsurface drainage treatment