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

Abstract

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.