Ironstone and red mud barriers to reduce lateral movement of soil phosphorus

Authors: SMITH, SAMANTHA - Louisiana State University; GASTON, LEWIS - Louisiana State University; BEASLEY, JEFFERY - Louisiana State University; WANG, JIM - Louisiana State University; Padilla, Joshua - Josh; WENGUANG, SUN - University Of Nebraska

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/12/2024
Publication Date: 7/23/2024
Citation: Smith, S., Gaston, L., Beasley, J., Wang, J., Padilla, J.T., Wenguang, S. 2024. Ironstone and red mud barriers to reduce lateral movement of soil phosphorus. Journal of Environmental Quality. 53(5):758-766. https://doi.org/10.1002/jeq2.20601.
DOI: https://doi.org/10.1002/jeq2.20601

Interpretive Summary: The movement of phosphorus from agricultural areas to the surrounding environment can cause major water quality problems. Permeable reactive barriers (PRBs) can be used to intercept agricultural runoff and remove phosphorus to prevent its movement offsite. However, the performance of PRBs to remove phosphorus strongly depends on the materials they are constructed from. In this study, we evaluated the suitability of ironstone or red mud for use in PRBs. We found that either material could bind approximately 930 mg of P per kg of ironstone or red mud. We then constructed model PRBs by mixing 90% sand, 9% soil, and 1% ironstone or red mud and packing the mixtures into columns. To evaluate their performance under operating conditions, we flowed phosphorus-containing water through the PRBs. We found that PRBs containing red mud removed 76% of the applied phosphorus whereas those containing ironstone only removed 13%, demonstrating that red mud was the superior material. Furthermore, we found that red mud increased the growth and phosphorus uptake of bermudagrass. This is important because the establishment of vegetation on top of PRBs prevents their erosion in natural settings, increasing their long-term performance. Overall, this work demonstrated that red mud is a promising material for use in PRBs for removing phosphorus in agricultural runoff.

Technical Abstract: Loss of phosphorus (P) in seepage may contribute to eutrophication of downstream water bodies. This research examined the potential use of pedogenic ironstone and untreated red mud (bauxite refining residue) as P sorbents in a permeable reactive barrier (PRB) to mitigate such loss. Effects on P sorption (batch), transport (columns), saturated hydraulic conductivity (KS) and growth of common bermudagrass (Cynodon dactylon; greenhouse) were examined. Both materials had sorption maxima of ~30 mmol P kg-1 or about five-times that of a P-enriched sandy soil, however, sorption by red mud greatly increased with decreasing pH. Elution of P from columns of ironstone, red mud and soil (diluted with nonreactive sand) showed similar retention of P by ironstone and red mud, but retention of P was greater for columns of soil + sand mixed with red mud than ironstone (76% vs 13%) due increased sorption at lower pH. Addition of ironstone to soil at 20% did not reduce KS, but addition of red mud even at 5% reduced KS by 40% at low ionic strength. However, addition of red mud increased bermudagrass growth and P uptake. Given long-term neutralization of red mud intermixed with an acidic soil and the accompanying increase in P sorption, it may be suitable as a P sorbent in a PRB if incorporated at a low rate and / or co-incorporated with a coarser material or flocculant.