Influence of iron-modified biochar on phosphate transport and deposition in saturated porous media under varying pH, ionic strength, and biochar dosage.

Phosphorus (P) is one of the essential nutrients required for plants; however, loss of phosphorus from agricultural areas results in water quality impairment. This research aims to investigate the transport and deposition of phosphate at different solution chemistries and phosphate-biochar dosages under (a) individual phosphate flow, (b) phosphate transport followed by biochar, and (c) co-transport of biochar-phosphate in saturated porous media. Breakthrough curves (BTCs) for phosphate were generated to understand the effect of pine raw biochar (BC) and iron-modified biochar (Fe-BC) on phosphate transport and deposition under varying solutions, pH (5.5 ± 0.1-10.5 ± 0.1), ionic strength (0-10 mM), phosphate (10-20 mg/L), and biochar dosages (100-200 mg/L) in saturated porous media. Results revealed increased deposition of BC and Fe-BC at high ionic strength (IS), i.e., 10 mM compared to 0 mM. The BTCs of phosphate (10-20 mg/L) transport at increasing IS showed delayed elute and long tailing curves compared to BTCs of tracer. Further, phosphate transport using BTCs in biochar-mediated saturated porous media was investigated at 10-20 mg/L phosphate, where maximum retardation (37%) was observed at pH 6.7 ± 0.1 and 0 mM IS due to the availability of active sites for 10 mg/L phosphate using Fe-BC than BC. The BTCs of phosphate transport at pH 6.7 ± 0.1 and 0-10 mM IS showed 37% and 40% phosphate deposition in Fe-BC-mediated columns for 0 mM and 10 mM, respectively, than BC-mediated columns. For BC, maximum phosphate adsorption was observed at pH 5.5 ± 0.1, whereas for Fe-BC, it was observed at pH 6.7 ± 0.1 at 10 mM IS. The least adsorption was observed at pH of 10.5 ± 0.1 for both BC and Fe-BC. Similar phosphate retardation BTCs for BC and Fe-BC at 10 mM were observed with adsorption of 40% phosphate for 100-200 mg/L biochar dosages. Besides, co-transport and deposition of biochar and phosphate, considering with and without ripening effect, reported high phosphate retardation using Fe-BC than BC at pH of 6.7 ± 0.1 and 10 mM IS due to chemical non-equilibrium and mass transfer. Taken together, iron-modified biochar (Fe-BC) showed significant adsorptive potential for phosphate management in saturated porous media. Overall, modeling of transport and deposition of phosphate and biochar are significant to understanding fate, nutrient mobility & management, biochar-phosphate interactions, and remediation designs in saturated porous media.